Green’s Dictionary of Slang

foreman n.2

[it stands at the forefront of the body]

the penis (cf. forewoman n.).

[UK]Farmer & Henley Sl. and Its Analogues.
[US]Maledicta IV:2 (Winter) 189: The reverse process of this [...] depersonification of the penis is its personification [...] as little man, foreman, him.