Green’s Dictionary of Slang

1. (Aus.) one who is in authority, the social or governmental elite.

1896		Clipper (Hobart, Tas.) 11 July 3/4: And arf the koshers of the town / Were marshalled on tbe square; / I can’t see ow we wants the foorce / To keep up order there.
1901		Sydney Sportsman (Surry Hills, NSW) 4 Sept. 2/6: Do you think the Koshers noise it? / I can tell you as they do. / Why, the thing’s kept dark for reasons.

2. a Jew.

1927

(con. WW1) P. MacDonald Patrol 166: ‘Close that stinkin’ mouth o’ yours, you sod of a bleedin’ Kosher! [...] You stinkin’ Jew!’.