To thwack is to hit something with a sharp heavy blow. Thwack the ball, a resounding thwack, thwacked him with a stick.
The word carries its own sound, which is why it appears in comics and in sports writing about a well-struck ball. It works as a noun for the sound as well as the blow. It is imitative, like whack and thump.
1verbthwackTo hit something with a sharp heavy blow.
“She thwacked the rug with a stick to beat the dust out of it.”
thwack is a blow.
An imitative word, made from the sound of the blow. Whack and thump were formed the same way.
Proto-Indo-European
*teh₂g-
English, today
thwack
The verb is probably:
Etymology from Wiktionary, CC BY-SA