Green’s Dictionary of Slang

foreman n.2

[it stands at the forefront of the body]

the penis (cf. forewoman n.).

News from the New-Exchange (1731) 12: [I]t is high Time he were some Way dispos’d of; for his Fore-Man is so flag and his Hams so feeble, that my Lady is constrained still to cry out; ‘Thy Finger again Jack’.
[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.