Green’s Dictionary of Slang

a woman (in any negative context).

c.1705		‘As Oyster Nan Stood by her Tub’ in Farmer Merry Songs and Ballads (1897) I 177: With that the Flat-capt Fusby smil’d / And would have blush’d, but that she cou’d not.
1764		K. O’Hara Midas II i: Neatest, Compleatest And sweetest Dear Fusby.