Green’s Dictionary of Slang

an onomat. term indicating a sudden noise or shock, typically an imitation of a handgun firing.

1902		Portsmouth Herald (NH) 29 Jan. 4/3: From all accounts, Richard Carle’s catch phrase ‘Ka-Pow, perfectly wonderful’ [...] promises to be even more popular than ‘is it possible’ of Burgomaster fame.
1920		‘Wonder What Ruth thinks About’ in Wellington Dly News (KS) 13 Sept. 3/4: Ah, here she comes, Babe, straight and fast *** / Ka POW!
1955		T. Southern ‘A South Summer Idyll’ in Southern (1973) 58: ‘Ka-pow! Ka-pow! Ka-pow!’ was what Lawrence said.
1977		J. Sayles Union Dues (1978) 150: You let things build up, like a pressure cooker, and then all of a sudden outa the blue – kapowie!
1987		S. King Misery (1988) 327: ‘But—’ KAPOW!
2001		(con. 1964–8) J. Ellroy Cold Six Thousand 396: A man laughed. A man went ‘Ka-pow!’.