1. to turn on someone you trust, or to have someone do the same to you.
2. team-killing, i.e. in a FPS. More specifically, this is the term used when one team-kills in Halo/Halo 2.
When you make a dealwith one of your best friends, like let your hair grow so you cut both have together a mulet for 1 day, and he (or his girlfriend making him look like a Simp) cuts his hair.
Thibaut: "You have cut your hair!? And the Mulet Haircut festival is in three months!"
Olivier: "Juliette wanted."
Thibaut: "What a BETRAYAL!"