Green’s Dictionary of Slang

roly-poly n.1

also roley-poley
[play on pudding n. (2) + ? SE roly-poly, stout, podgy]

the penis.

[UK]Partridge DSUE (1984) 985/2: mid-C.19–20.
[Aus](con. 1940s–60s) Hogbotel & ffuckes ‘Heigh Ho Says Rowley’ in Snatches and Lays 31: B is the bugger who revels in it / With a Roley-poley, up ’em and stuff ’em.