The name Jareth was an invention between Jim Henson, Brian Froud, Dennis Lee and Terry Jones for the movie, but I can’t find anything that specifies which of the four actually came up with the name, although Terry Jones was the main draft writer for the script. After some light research, it is suggested that Jareth is a compound name between Jared and Gareth. https://www.behindthename.com/name/jareth, https://www.ancestry.com/first-name-meaning/jareth. One thing that’s repeated but never substantiated, is that Jareth means “gentle”, but I couldn’t find anything to substantiate this claim. This is in conjunction with a bunch of other low quality information, or just blatant misinformation surrounding it.
https://www.blueletterbible.org/lexicon/h3382/kjv/wlc/0-1/ Jared (ירד), hebrew for “to descend/descender” https://www.abarim-publications.com/Meaning/Jared.html, is the name of the son of Mahalaleel, and is the father of the second Enoch as mentioned in Genesis chapter 5. The James Strong Exhaustive Concordance (1990), pg 120 of the Hebrew dictionary, states the following on Jared: “yarad, yaw-rad’; a prim root; to descend (lit. to go downwards; or conventionally to a lower region, as a shore, a boundary, the enemy, etc.; or fig. to fall); caus. to bring down (in all the above applications): - (come, go, etc) down {340x}, descend {18x}, variant {2x}, fell {2x}, let {1x}, abundantly {1x}, down by {1}, indeed {1x}, put off {1x}, light off {1x}, out {1}, sank {1x}, subdued {1x}, take {1x}. Yarad means “to descend, to go down, come down.” (1) Basically, this verb connotes “movement” from a higher to a lower location. (1a) In Gen 28:12, Jacob saw a “ladder set up on the earth, and the top of it reached to heaven: and behold the angels of God ascending and descending on it.” (1b) In such a use, the speaker or observer speaks from the point of destination, and the movement is downward” toward him, and the movement is “downwards” toward him. (1c) Thus one may “go down” below or under the ground’s surface: And the damsel was very fair to look upon, a virgin, neither had any man known her: and she went down to the well, and filled her pitcher, and came up” (Gen 24:16). (1d) The speaker may also speak as though he stands at the point of departure and the movement is away from him and “downwards.” (1d1) Interestingly, one may “go down” to a lower spot in order to reach a city’s gates (Judg 5:11) or (1d2) to get to a city located on a lower level than the access road (1 Sa 10:8)–(1d3) usually one goes up to a city and “goes down” to leave a city (1 Sa 9:27). (1d4) The journey from Palestine to Egypt is referred to as “going down” (Gen 12:10). This reference is not a movement in space from a higher to a lower spot; it is a more technical use of the verb. (2) Yarad is used frequently of “dying.” (2a) One “goes down” to his grave. Here the idea of spatial movement is present, but in the background. (2b) This “going down” is much more of a removal of the world of conscious existence: “For the grave cannot praise thee, death can not celebrate thee: they that go down into the pit cannot hope for thy truth. The living, the living, he shall praise thee...” (Is 38:18-19). (2c) On the other hand, “going down to the dust” implies a return to the soil—i.e., a return of the body to the soil from which it came (Gen 3:19). “All they that go down to the dust shall bow before him...” (Ps 22:29). (2d) There is also the idea of the “descent” of the human soul into the realm of the dead. When Jacob mourned over Joseph whom he thought to be dead, he said: “For I will go down into the grave unto my son mourning” (Gen 37:35). (2e) Since one can “descend” into Sheol alive as a form of punishment (Num 16:30), this phrase means more than the end of human life. (2f) This meaning is further established because Enoch was rewarded by being taken off the earth: “And Enoch walked with God: and he was not; for God took him.” (Gen 5:24); he was rewarded by not having “to descend” into Sheol.
Like with Jareth, it has been repeated, yet never substantiated, that Gareth means “gentle”. If I had to guess where this comes from, it is likely from poor phonetic translation. The first recorded instance of the name Gareth is from Thomas Malory’s Le Morte d'Arthur (1485), the brother of Sir Gawain, and the main character of Book VII. Sir Kay refers to Sir Gareth as Beaumains: “I shall give him a name that shall be Beaumains, that is Fair-hands” Pg 178. “My name is Gareth of Orkney, and King Lot was my father, and my mother is King Arthur’s sister, her name is Dame Morgawse, and Sir Gawaine is my brother, and sir Agravaine and Sir Gaheris, and I am the youngest of them all.” Pg 193 (Book VII).
It also says here https://nightbringer.se/the-legend-of-king-arthur/arthurian-characters/b-persons-in-the-arthurian-legends/beaumains/ that Beaumains shares a connection to “Fair Unknown” whose name was hidden and then revealed to be Gunglain, A son of Sir Gawain and Blanchemal the Faery (Blanch means bleach. Emal is supposedly a suffix to indicate a process https://en.wiktionary.org/wiki/emal, so her name would translate to Bleacher). This is also supported by the book “The Great Book of King Arthur & His Knights of the Round Table” (2022)”, by John Matthews, which contained a chapter titled: The Adventure of the Fair Unknown. Called Libeaus Desconus by King Arthur, and his quest to the Desolate City and his fight with the enchanter, Mabonagrain, there. “I tell you now that your true name is Guinglain. You are the son of Gawain and Blachemal the Faery” Pg 97. However, it should also be noted that in the chapter “The Wedding of Gawain and the Lady Ragnall”, it says that Lady Ragnall was the mother to Gunglain. “Gawain and Ragnall were joyful together thereafter and in due time the lady bore a son whom they named Gunglain” Pg 305. Perhaps it suggests that Blachemal is a nickname, and Lady Ragnall is her full name, since Ragnall was a faery, which is the defining trait shared between the two characters, so they could be the same person. He also meets a princess called White Hands on his adventure. “This place is a dreadful place. Within it lives a damsel of great beauty called the Maiden of the White Hands. She is schooled in the seven liberal arts, knows the mystery of the stars, and the ways of enchantment." pg 92. Her appearance in the story is pivotal to the plot, as she is the love of Guinglain, and behind all the events of the plot: “Then she confessed that she had always known Guinglain would come, that he would leave her and return. Indeed, the whole of his great adventure had been her doing. She it was who had sent Helie to Arthur’s court to ask him for help. It was her voice that announced his true name after he had braved the serpent’s kiss. ‘You see, my love I have been waiting for you this long time’” Pg 100. This indicates some symbolic connection between the two subjects. Either way, this has some symbolic connection to Gareth’s story in Malory’s work, and Fair Unknown and White Hands are both connected to the meaning of Gareth and his nickname of Beaumains. However, I’m unable to find anything at all on the name Guinglain, so there I’m completely unsure of any meaning. Here https://www.behindthename.com/name/gareth it proposes that the origin of Gareth can be either Gwrhyd (valor), or Gwairydd (hay lord). However, when I look for the suffixes of “hyd” by itself, the definition seems less sure. When I look for hyd, all I can find is the prefix hydr https://www.etymonline.com/word/hydraulic, which is derived from a Greek origin, not Welsh. However, on this site https://wtname.com/gareth/ it asserts that “gar” means “spear” and “et” means “hand/rule”, and that Gareth descends from gwalch. “It is derived from the Welsh words “gwalch” meaning “hawk” and “ur” meaning “hand” or “rule”. Therefore, the name Gareth can be interpreted to mean “spear rule” or “ruler with a hawk-like vigilance”. Although the site lacks proper citations for these claims, it asserts the “reth” of Gareth to mean “rule”, which corresponds with “rydd” meaning “lord”, which reassures me that the name relates to rulership. As for the first syllable, Gar is straight from the old English Gar (ᚸ), which comes from old Germanic Gebo (X) of the elder futhark runes. I’m also reminded of the name Edgar, which means “great spearman”, or “great gift”. https://www.behindthename.com/name/edgar “Derived from the Old English elements ead "wealth, fortune" and gar "spear"”. So from that I’m reassured that gar is Anglo-Saxon (which is Old English), and means “spear”, and reth, which comes from Welsh “rydd”, means “hand/rule/lord”. Gareth would then be itself a compound of Welsh and Anglo-Saxon, and means several things: “spear in hand”, “spear ruler”, “spear lord”. It could even have a meaning like “gift in hand”, “gift ruler”, or “gift lord” based on the older definitions. This also coincides with Sir Kay’s nickname of Sir Gareth being Beaumains, which also refers to the hands, which is enough to make me confident that this is the correct meaning.
If I take this into consideration, I can thereby assume that Jareth, as a compound, means “descended lord”, or “lord of the descended (underworld)”. The Labyrinth that Jareth rules over as the Goblin King is the Underworld, the realm of imagination, which can only be entered by traveling south (aka: descending). This also ties into the Hebrew use of descent into Sheol, which was the underworld.
As an ending thought, this word is a strong example of the lingual connection between Hebrew and Welsh that is often overlooked, but that’s a much more intricate topic to get into. If anyone reading, especially those with a better understanding of Hebrew or Welsh than me, wants to provide more insight into this, please share.