Green’s Dictionary of Slang

(N.Z. prison) a prison-made weapon using batteries or cakes of soap tied inside a sock .

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 59/1: dolly n. 3 a weapon made from a pool ball, D-sized batteries, or cakes of soap tied in a sock, a stocking, or a pillowcase. [poss. from colloquial 'dolly' = a pool ball.