Green’s Dictionary of Slang

mettle n.

semen.

1608		Dekker Honest Whore Pt 2 (1630) I ii: [I’ll] try what mettle is in his new bride, if there be none, we’ll put in some.
1615		J. Swetnam Araignment of Lewd, Idle, Froward and unconstant Women IV iii: A Ladie [...] Will sooner meet a Tinker in the street, / And try what Metall lyes within his Budget.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
c.1800		Burns ‘Citadel’ Merry Muses of Caledonia (1843) 65: His metal flew about like hail / In the centre of my citadel.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.