Green’s Dictionary of Slang

boss n.1

[SE boss, a swelling or protuberance]

a fat woman.

[UK]Lyly Euphues and his England (1916) 103: If she be well set then call her a boss, if slender a hazel twig.
Marlowe Tamburlaine Pt I III iii: zab.: Base concubine, must thou be plac’d by me, That am an Empresse of the mighty Turke? zen.: Disdainful Turkess and unreverend Boss!
[UK]R. Cotgrave Dict. of Fr. and Eng. Tongues n.p.: A fat boss, Femme bien grasse et grosse; une coche.