A Weave Whack is when someone has in synthetic braids/weave and turns expectantly and whacks/whips you in the face with said weave/braids.
Me: Yeah and so thats when Keisha-
Person: HEY SHAUNA
Shauna: *Turns real fast and whacks in face with weave*
Me: Damn Shauna you just weave whacked me
Shauna: Haha my bad!