Most hilarious line ever, and a way to vent anger when your friend rejects an outing (usually a pathetic excuse) cause your bored as fuck and stuck at your house all weekend. Originated from the movie Mean Girls when Karen rejects hanging out with Regina because she called her a slut.
The famous quote:
Karen: I can't go out, *cough cough* i'm sick.
Regina: Boo, you whore! *hangs up*
Another example of an annoying whoebag who ruins your weekend:
Girl 1: Hey, wanna hang out tonight? I'm so bored.
Girl 2: Nahh, i'm too tired... srry. mybe ltr ;)?
Girl 1: boo, you whore!