What is Whack?
Whack is (verb) to hit hard, making a loud noise She whacked her head on the low doorway. He whacked the ball hard with his bat. (noun) 1. a hard, noisy blow He gave the ball a whack and it went over the wall. 2. have a whack (at it)! try to do it (informal.) 3. an amount of money We each paid the full whack.
source: Easier English, Student Dictionary Upper Intermediate Level