Any character created or written into fan fiction, that clearly does not belong, but is there to sublimate the authors sexual desire for an actual well established character.
Me: I don't need to read about your Harley Quinn.
This term originates from Batman's Harley Quinn, who first appeared in Batman the animated series and gained so much popularity that she was placed into the comic books.
When a character which originates from a cartoon gets placed into the comic books, or becomes canon.
Are they really going to Harley Quinn that character? He's annoying enough in the cartoons.
Harley Quinn: You left em in the car Puddin!