thwack definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
to strike with something flat; whack
Origin: probably echoic
a blow with something flat
thwack definition by American Heritage Dictionary
transitive verb thwacked, thwack·ing, thwacks
To strike or hit with a flat object; whack.noun
A hard blow with a flat object; a whack.