Green’s Dictionary of Slang

polly n.1

[poll n.2 or Polly, nickname for Mary = moll n. (1)]

1. a mistress, a prostitute who lives with a man.

[UK]‘The Bilk’ in Randy Songster in Spedding & Watt (eds) I 208: The tailor was quite overjoyed that he had got a polly, / He took her up an alley, quite safe from all annoyment, / And, promising to pay her well, he had his full enjoyment.
[UK]A. Binstead More Gal’s Gossip 1: Only the most impulchritudinous pollies and mediaeval tabbies still cling to the powder-puff.

2. a woman.

[UK]Paul Pry 15 Jan. n.p.: R—d F—lf—d, alias [...] alias the Clare market ‘Adonis’ [...] not to be seen so frequently ‘doing a drain’ with the ‘Pollys’ in Fleet street.
[UK]M.E. Braddon Dead Men’s Shoes II 251: I’m glad it isn’t any other kind of Polly you tremble for, Dick.
[UK]A. Binstead More Gal’s Gossip 85: Charlie said might as well rejoin the pollies on the coach.