Green’s Dictionary of Slang

1. (also browney) a child.

1929		C.G. Givens ‘Chatter of Guns’ in Sat. Eve. Post 13 Apr.; list extracted in AS VI:2 (1930) 131: Brownie, n. A child. Also a type of machine gun.
1949		Monteleone Criminal Sl. (rev. edn) 34: browney A boy; a young tramp.

2. a machine gun [the Browning machine gun].

1929		see sense 1.
1950		Goldin et al. DAUL 35/1: Brownie. A heavy machine gun.