A "good whack" means a strong hit or strike that is satisfying.
Sentences with «good whack»
If you don't have any of the above just put the ice in a resealable plastic bag then put that in another larger resealable plastic bag and give it a few good whacks with a rolling pin until the ice is in tiny pieces. (holajalapeno.com)
I love that you've used a really good whack of ginger in there, no pussy - footing around with teaspoons. (isachandra.com)
Carefully turn the bomb over onto the plate, giving it a couple of good whacks on the counter to get it out of the bowl. (allroadsleadtothe.kitchen)