To strike or hit with a flat object; whack.(verb-transitive)
A hard blow with a flat object; a whack.(noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "thwack" in a sentence
"What they forgot to take into account, however, is that you do not hear the primary charge of a rifle being fired if you are more than 100 metres from it, you only hear the "thwack" or "crack" of the bullet passing overhead, followed by a MUTED primary charge thump (weapon firing)."
"A good meaty "thwack" means the thrower needs to go hide for a few seconds."
"At the last moment she turned parallel to the chukar's line of flight and hit it from behind with the solid "thwack" of a large-caliber bullet striking flesh."