Green’s Dictionary of Slang

whop! excl.

also whap!, wop!

echoic of the sound of a sudden blow.

[UK]W. Perry London Guide 33: Should the pair come down whop, it is far the better for the thieves.
[US]T. Haliburton Clockmaker I 105: At other times they go whap onto a quicksand.
[US]‘Jonathan Slick’ High Life in N.Y. I 207: I went whop down on my knees.
[US]M.L. Byrn Adventures of Fudge Fumble 42: ‘Whop’ went ole Johnny’s foot on the piazza floor.
[Aus]Bulletin (Sydney) 24 Apr. 5/4: The springs of tghe buggy at length collapsed and wop went Lucas.
[US]J. London ‘Bald-face’ in Aegis (Oakland High School) 6 Sept. 1–2: Next thing I knows — whop, I comes up against something in a tangle of wild blackberry bushes.
[UK]P. Larkin letter 31 Dec. in Thwaite Sel. Letters (1992) 29: Whop! I’ve just been struck dead for blasphemy.
[US]B. Schulberg On the Waterfront (1964) 250: I step inside the jab — whop! with a right.
[US]V.E. Smith Jones Men 9: Joe slapped him upside the head with his pistol. Whop!