Charlie Jane Anders recently tweeted a New Year's wish that everyone "retire the term 'Mary Sue,' which at this point just means "female hero" with negative connotations."
A "Mary Sue" has traditionally been genre fiction shorthand for a wish-fulfillment character. I believe it may have originated in critiques of fan fiction, which at times featured characters who were thinly-veiled surrogates for the writers' fantasies of interacting with their literary heroes. Often, these characters were ridiculously skilled and smart, drawing the appreciation of Kirk, Harry, Aragorn, or other franchise star.
I've used the term in my writing groups, not so much as an accusation but in discussions of characterization. I also remember when a beta reader suggested that my first novel-length project flirted with wish fulfillment, as its main character shared some meaningful aspects of my life. I had believed I was merely "writing what I knew" and found this critique dismaying and insulting. Being told you've written a Mary Sue character is essentially being accused of not doing your job as a writer.
I had no idea the term had acquired a broader, gendered connotation before I read that tweet. Then I went back and read Anders's article about how Rey from The Force Awakens has been accused of being a Mary Sue, simply because she's a capable female sharing screen time with the characters of the original trilogy.
What the “Mary Sue” thing shows—other than that people will find any craptastic excuse to tear down female characters—is that memes have a decay rate.
This is a startling idea - that a term that helped me understand characterization (and improve my own writing) can be shot out from under me by people who approach fiction with a radically different agenda. Anders isn't describing a natural process of decay and replacement, but a purposeful appropriation of terms. "Mary Sue" may not be the meme worth fighting for, but it once served a useful function. What will take its place, and how long before that, too, is ceded to people determined to Make Science Fiction Great Again?