Green’s Dictionary of Slang

1. echoic of a sudden, usu. violent action.

1843		T. Haliburton Sam Slick in England I 222: He’ll stop short, and pitch Tom right over his head on the broad of his back, whap.
c.1860		‘New Fashioned Pettitcoats’ in C. Hindley Curiosities of Street Lit. (1871) 147: Down he went like a donkey wop bang on his nose.
1977		R. Coover Public Burning (1998) 55: We had two kids — whap bang! — before she even knew what hit her, and for a couple of years there it was pretty fantastic.
1999		M. McDonough ‘Skull in Connemara’ in Plays 118: [He] doesn’t bat an eye at a blood-soaked man standing whap-bang in front of the feck-brained fool .
2008		J.M. Jones Black Whole 76: [...] with my face smashed up against the windshield, hearing a whap-bang, knowing it was the gas-tank or maybe the crumpled fuel line catching.

2. attrib. use of sense 1.

1989

New York Mag. 17 July 45/2: The trouble with Burton is that he isn’t very good at whap! bang! violence.