thwack

Short definition from Wiktionary

Verb

1. 1
   To hit (someone or something) hard, especially with a flat implement or a stick; to thrash, to whack.
2. 2
   To drive or force (someone or something) by, or as if by, beating or hitting; to knock.
3. 3
   To pack (people or things) closely together; to cram.
4. 4
   To decisively defeat (someone) in a contest; to beat, to thrash.
5. 5
   To crowd or pack (a place or thing) with people, objects, etc.
6. 6
   To fall down hard with a thump.
7. 7
   To be crammed or filled full.
8. 8
   Of people: to crowd or pack a place.

Noun

1. 1
   An act of hitting hard, especially with a flat implement or a stick; a whack; also, a powerful stroke involved in such hitting; a blow, a strike.
2. 2
   A dull or heavy slapping sound.

Interjection

1. 1
   Used to represent the dull or heavy sound of someone or something being hit or slapped.