Green’s Dictionary of Slang

echoic of the sound of a sudden blow, used to indicate suddenness, immediacy.

1960		J. Brosnan Long Season (2018) 258: ‘You could throw him ten off-speed pitches in a row, and then come in with a good hummer. Whomp!’.
1972		G.V. Higgins Friends of Eddie Coyle 71: All of a sudden, whomp, he’s got that shot down and the beer running right after it.
1987		J. Jerome q. in Firestone Swing, Swing, Swing (1993) 238: ‘ And then someone playing trumpet stood up and—whomp!—it was the same thing. The crowds were crazy. Crazy!’.
1999		Guardian Rev. 15 Oct. 8: She takes her time before – whump! – a great thud, and the door shudders before thick, dark blood trickles beneath it.