#it's not just “female character I don't like”#it's not just “self insert” (though there's usually overlap)#it's not just “poorly written” (though they usually are)#and the male ones were called “Gary Stus”#and words used to mean something dammit
It meant an original character, often a self-insert, who was created as wish fulfillment to the point where it would alienate any other potential readers that weren't the author. The character would not fit aesthetically into the pre-established world in which they were put. The character would have abilities that disregarded the previously established rules of said world, or were insanely overpowered to such a degree that it dissolved any tension there could be in the plot. And most importantly, any flaws they might have were done in such a way where they could be made into something cool or mysterious, if they were written with flaws at all.
As you can guess, these were most often written by children and/or inexperienced writers. And in retrospect, like yeah, the characters are not fun to read and they're really annoying, but there's far worse things than being merely annoying. Like, for instance, insisting Rey from Star Wars is a Mary Sue because she's powerful enough for Luke to be understandably freaked out (the stakes need to be raised, after all) and then having a decade-long tantrum about it.
I feel the same way I do towards dude nerds who throw around "Mary Sue" without having ever spent a day in the fanfic mines as I do to anyone who uses "based" and has never listened to Lil B. How dare you. I am going to beat you to death.