Jap Slap

It means multiple things. Basically, its a WWII reference, meaning:

1. Karate chop. Japanese were known by soldiers for their superior hand to hand combat skills, particularly in karate.

2. To strike a person unexpectedly, similarly to a sucker punch.

3. Loosely, it refers to an unexpected attack or ambush.

Wow! Did you see Tom get into a fight, yesterday? He hit that guy with a vicious jap slap and almost knocked him out.