Green’s Dictionary of Slang

beastie n.

the penis.

[UK]A. Warner Sopranos 213: That was just classic, starkers wi his beastie sticking out.

In compounds

beastie boy (n.)

(N.Z. prison) a rapist.

[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 15/1: beastie boy n. a rapist.