Green’s Dictionary of Slang

echoic of the sound of a sudden blow.

1818		W. Perry London Guide 33: Should the pair come down whop, it is far the better for the thieves.
1827		J. Wight More Mornings in Bow St. 231: He suddenly received ‘a squashy, well-damaged St Michael’s [orange], vop in his right ear’.
1836		T. Haliburton Clockmaker I 105: At other times they go whap onto a quicksand.
1841		Whip & Satirist of NY & Brooklyn (NY) 25 Dec. n.p.: Wap! he received a flush left-hander over the blinker.
1844		‘Jonathan Slick’ High Life in N.Y. I 207: I went whop down on my knees.
1865		M.L. Byrn Adventures of Fudge Fumble 42: ‘Whop’ went ole Johnny’s foot on the piazza floor.
1880		Bulletin (Sydney) 24 Apr. 5/4: The springs of tghe buggy at length collapsed and wop went Lucas.
1901		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.
1941		P. Larkin letter 31 Dec. in Thwaite Sel. Letters (1992) 29: Whop! I’ve just been struck dead for blasphemy.
1955		B. Schulberg On the Waterfront (1964) 250: I step inside the jab — whop! with a right.
1964		D. Gregory Nigger 34: Whop. Momma spun across the front room, back toward the kitchen, like a drunk. Whop. Big Pres had the belt out again.
1974		V.E. Smith Jones Men 9: Joe slapped him upside the head with his pistol. Whop!