Green’s Dictionary of Slang

(orig. US) an excl. used to describe the force of a sudden impact.

1929		P.F. Nowlan in Wash. Post 7 May 16/3: Ahead of me was one of those golden dragon Mongols, with a deadly disintegrator ray... Br-r-rr-r-z-zzz-zap.
1968		Maclean’s (Toronto) Mar. 77: Bang! Zap! Pow! With flashing laser beams and cracking doomsday machines, the deadly-serious superheroes swarmed out.
1974		(con. late 1940s) E. Thompson Tattoo (1977) 576: Then, just when it seemed to be gettin all right, you know. Zap! This shit happened.
1982		P. Redmond Tucker and Co 83: A cracking shot at the back of Pongo Yates’s head. Zap.
1998		L. Stringer Grand Central Winter (1999) 15: To express displeasure, he often put on a disappearing act. Cross a certain line with him and ZAP! You’d be left confronting an impenetrable void.