Green’s Dictionary of Slang

(Aus./US) a police officer.

1889		Dead Bird (Sydney) 21 Dec. 8/1: The race is not always to the swift. Always find out who's a jigger.
1895		J. London ‘’Frisco Kid’s Story’ in High School Aegis X (15 Feb.) 2–4: I took yer fer a fly cop, an’ I’m onto meself fer a jigger w’en it comes to dem people.
1932		(con. 1910s) J.T. Farrell Young Lonigan in Studs Lonigan (1936) 70: ‘Jiggers!’ yelled Johnny O’Brien. Across the street [...] they saw TB’s old man, a tough red-moustached, Irish policeman. They legged it.