1. To strike one very forcefully. He whacked me on the head for speaking out of turn. Stop swinging that tree branch around. You're going to whack someone with it if you're not careful!
2. slang To murder or arrange the murder of one. Used especially in relation to organized crime. The guy is such a psycho. He'd whack you just for looking at him the wrong way! The mob whacks anyone who testifies against them in court.
See also: whack