Green’s Dictionary of Slang

1. (US, also jap-jump) to attack without warning, esp. of street gangs.

1942		in W.C. Fields By Himself (1974) 186: The fellows [...] in Pearl Harbor [...] were caught napping, by the Japs Japping.
1959		W. Miller Cool World (1965) 14: One time 3 Wolves japped me in my own bilding [sic].
1967		P. Thomas Down These Mean Streets (1970) 53: Look out, ya gonna get japped.
1975		E. Torres Carlito’s Way 90: How could you jap us like this.
1989		H. Rawson Dict. of Invective (1991) 216: To pull a Jap or to Jap [someone] became synonymous with a sneak attack of any kind.
1990		(con. early 1950s) J. Ellroy L.A. Confidential 213: High-spirited darkies japping the search teams.
1992		R. Price Clockers 424: Maybe Champ got this kid Strike to go in and jap his old buddy.
1998		at First Amendment Exercise Machine 14 Mar. 🌐 When the year 2002 A.D. (After Deception) arrives, with the niggers still niggerizing and the jews still jewing, the Japs japping, etc. lots of Armageddonists will have to sober up and resume waiting for Jesus.
2010		(con. 1973) C. Stella Johnny Porno 86: The motherfucker japped me. The guy hit me when I wasn’t looking.
2021		J. Ellroy Widespread Panic 283: Somebody Jap-jumped me. A dogpile ensued.

2. (US campus) to steal.

1967–8

Baker et al. CUSS 129: Grease and jap Steal.

3. (US) to undermine someone’s plans or efforts, to surprise.

1957		W. Bernstein ‘The Cherubs Are Rumbling’ in Short Gang Delinquency and Delinquent Subcultures (1968) 38: ‘They started it,’ [fight] one of the others said. ‘They japped us,’ a third boy said, meaning that the Cherubs had taken them by surprise.
1966		Current Sl. I:3 5/1: Jap, v. Surprised [...] To make a zero on an exam.

4. (also jap out) to back down, to renege on an appointment.

1966		Current Sl. I:3 5/1: Jap out, v. To fail to keep an appointment or date.
1967–8		Baker et al. CUSS 144: Japped Treated unfairly on an exam. Japped out Turned down when asking for a date.
1986		Eble Campus Sl. Oct. 4: jap – back out on an obligation: Marie was supposed to go to the mixer with us, but she japped.
2012		(con. 1971) M.F. Armstrong They Wished They Were Honest 98: RATNOFF: I’ll have, I’ll have all of it. I’m not going to Jap you for $3,500.

5. to swindle, to be cheated.

1967–8		Baker et al. CUSS 144: Japped Treated unfairly on an exam.
1986		S. King It (1987) 351: I oughtta take it back [...] It’s supposed to sleep but it doesn’t. I got japped.

6. to hit, to knock.

1981

S. McAughtry Belfast 6: Six pints of Double in the 1930s, and you were liable to jap your brains out against a lamp post on the way home.

7. (US black) to punch.

1995

L. Stavsky et al. A2Z.