Green’s Dictionary of Slang

siss n.1

1. a large, fat woman.

[UK]J. Taylor ‘An Armado’ in Works (1869) I 82: The Drunken Siss, a great Ship.

2. a prostitute.

[UK] in D’Urfey Pills to Purge Melancholy III 55: She had a daughter her name was Siss [...] She kept her at Home for to welcome her Guest.