Suggests juxtaposition of two things; one of them is usually a person or character. The word preceeding the exclamation point is never something daily. It's always a costume or disguise or new personality or something.
Originated in the fanfiction community, specifically among the AU (alternate universe) crowd.
The best part about going to the rodeo was seeing Cowboy!Todd in his outfit.
I don't know what's been up with you lately, but Bitch!Lacey needs to be fragged.