Green’s Dictionary of Slang

1. (Aus.) to flog. thus breeching, a flogging.

1895

Crowe Aus. Sl. Dict. 12: Breeching, a flogging.

2. (UK Und.) to steal from someone’s trousers.

1935		Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 2: Breech: Steal from the person.
1936		W.A. Gape Half a Million Tramps 27: Well, you’ll have a better bed to-night, unless you can prove it wasn’t you who ‘breeched’ that sailor and left him fast asleep in bed.
1950		P. Hoskins No Hiding Place! 189/2: Breeching Robbing.