Green’s Dictionary of Slang

(Aus./Irish/US) a derog. term for a Japanese person.

1900		Bulletin (Sydney) 26 May 12/3: He enjoyed as much distinction as the Jappies.
1904		Eve. Teleg. (Dundee) 17 Feb. 2/5: [headline] How jappy Goes to War.
1916		All Abaht It Nov. 11: There’s our little baby Jappy, / Whom the girls think really nice.
1922		Joyce Ulysses 406: Jappies? High angle fire, inyah! Sunk by war specials.
1927		Dundee Courier 29 June 9/4: On the beach they met a Jappy.