Green’s Dictionary of Slang

underput n.

[SE put under; i.e. the position of sexual intercourse]

a mistress.

[UK]Middleton Michaelmas Term III i: salewood: Is she but your underput, Master Lethe? lethe: No more [...] I may grace her with the name of a courtesan, a backslider, a prostitution, or such a toy; but [...] ’tis but a plain pung.