Green’s Dictionary of Slang

1. one who carries a cosh n.

1889		Clarkson & Richardson Police! 348: ‘Coshers’ and ‘trippers’ or ‘picking-up molls’, are vile men and women who travel from town to town.
1895		J. Caminada Twenty-Five Years of Detective Life I 164: The ‘coshers’ set upon him and rob him of his watch.

2. (Aus., also cosher-man, kocher, kosher) a police officer; also attrib.

1900–10		Stephens & O’Brien Materials for a Dict. of Aus. Sl. [unpub. ms.] 49: COSHER OR KOSHER: Sydney and Melbourne slang a policeman, variety of copperman.
1900		Truth (Sydney) 14 Oct. 5/2: Sez the kosher, ‘Wares the good / Of smashin’ glass to get in chokey?’.
1902		Truth (Sydney) 31 Aug. 8/3: ‘Drive me, cabby, / To the Central kosher place: /I’ve been biffed & robbed & bullied.
1902		Truth (Sydney) 31 Aug. 8/3: But the koshermen they nose him. / And they clears him, don't you see?
1910		Truth (Perth) 13 Aug. 4/6: Them kochers don’t see nothin’ / When they should do, I expect.
1913		Truth (Sydney) 16 Feb. 7/8: She changed, her mind— / (When she sees the Kosher standin’, / She was not that way inclined).