>someone explain to me how the fuck is cuck rule breaking. I thought it was a common word?
Only since about 2014, around the time of Gamergate - that's when it became 4chan/8chan's go-to insult, and was overused to death in the years after that. It goes hand-in-hand with overuse of wojak/pepe reaction images, 2016 election era, "kekistani", culture war crap, etc.

Before that - and still the case outside of shitty imageboards - you almost never saw the term outside of rare discussions relating to the fetish. I think the imageboard-specific usage is well within Rule 8 territory