HFY is an abbreviation for "Humans, F**k Yeah !" - it's one of several related genres, including "Humans Are Space Orcs" and "Space Australia" which all developed immensely in Tumblr and Reddit long before AI's came along to try to imitate the genius ideas and memes they spawned, and which built up layer over layer.
Here's one of my faves....
https://kalira.tumblr.com/post/656039948779159552/demon-you-have-summoned-me-mortal-what-deal-do