The act of hardcore flirting, sleeping with, or dating the subject of your friend's affection. Usually occurs after you and your friend discussed the means of who would chase said subject, and who would lay low.
(usually occurs in the female sex)
Girl 1: So last night, I hooked up with Mark.
Girl 2: I thought Stephanie was into Mark? Didn't she talk to you about it?
Girl 1: Yeah, but I went ahead anyway.
Girl 2: If that's not slutting, I don't know what is.