Thwack is the sharp, solid sound something makes when it hits with force, usually something flat or heavy striking a surface. It's that satisfying thwack when a well-hit baseball connects with the bat, or the thwack of a book dropping flat onto a desk.

The word captures both the sound and the feeling of impact. When you hear something go thwack, you know it hit hard and square, not a glancing blow or a gentle tap. A flyswatter makes a thwack against the wall. A beaver's tail makes a thwack when it slaps the water's surface to warn other beavers of danger.

You can also use thwack as a verb: to thwack something means to hit it with that same sharp, solid force. Someone might thwack a ruler on the desk to get the class's attention. The word is onomatopoeia, meaning it sounds like what it describes, just like buzz, splash, or crunch.