|is the sound made by a ray-gun when it's fired, if you've not had occasion to notice. But a Zap-gun is a water-pistol, or sometimes a toy ray-gun. Martin Alger explains the ultimate source thus:
"At the Torcon they showed an atomic energy movie and a lot of the neofen were milling around during the showing. I asked Ben Singer if he were 'bored because nobody in the film has pulled a raygun and gone Zap! Zap!?'... I never heard the term used before this, so I guess that was the source."
Apparently, unbeknownst to any, a reporter was standing in the neighborhood, for the Toronto Morning Star headlines its convention report "Zap! Zap! Atomic Ray Passe with Fiends!" And, as Martin says, fans were delighted with this and the term caught on from there.