Green’s Dictionary of Slang

(UK Und.) a trouser pocket.

1879		‘Autobiog. of a Thief’ in Macmillan’s Mag. (London) XL 502: I went to the Steel [...] having a new suit of clobber on me and about fifty blow in my brigh (pocket).
1887		‘Dagonet’ ‘A Plank Bed Ballad’ in Referee 12 Feb. n.p.: One day I’d a spree with two finns in my brigh, / And a toy and a tackle – both red ’uns.