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
Have you seen Chloe Sullivan from Smallville? She recently got Harley Quinned into the comics.
Are they really going to Harley Quinn that character? He's annoying enough in the cartoons.