Green’s Dictionary of Slang

forge n.

[where the male ‘rod’ is softened]

the vagina.

[UK] ‘A Lusty Young Smith’ in Farmer Merry Songs and Ballads (1897) I 175: When to him a Buxom young Damsel came smiling, / And ask’d if to Work at her Forge he wou’d go; / With a rub, rub, rub, rub, rub, rub in and out, in and out ho.
[UK]Farmer & Henley Sl. and Its Analogues.