Whack means to hit something hard and suddenly, usually making a sharp sound. You might whack a piñata with a stick at a birthday party, or accidentally whack your shin against a table leg. The word captures both the action and the sound: that quick, solid thwack when something strikes something else.

As a noun, a whack is the hit itself or the sound it makes. You might take a whack at splitting firewood with an axe, or hear the whack of a baseball bat connecting with a ball.

In casual speech, people use “out of whack” to mean something isn't working right or is off-balance. If your bicycle chain keeps falling off, you might say the gears are out of whack. When a clock runs too fast or too slow, its timing mechanism has gotten out of whack.

The word has an informal, slightly comic quality to it. You wouldn't say a surgeon whacks during an operation, but you might say a frustrated cook whacks a stubborn jar lid to loosen it, or a frustrated gardener whacks weeds with a hoe.