Green’s Dictionary of Slang

(US) a derog. term for a black person.

1909		‘The Lang. of Crooks’ in Wash. Post 20 June 4/1: [paraphrasing J. Sullivan] A Jap or Dingo is the term used for negroes, as is also honey .
1928		R. Fisher Walls Of Jericho 297: Synonyms of Negro [...] : ink, jap, jasper.
1949		Monteleone Criminal Sl. (rev. edn).
1950		Goldin et al. DAUL 110/1: Jap. (Chiefly among carnival and transient grifters) A Negro.
1992		Bentley & Corbett Prison Sl. 55: Jap A black person.