The Collaborative International Dictionary of English v.0.48:

Doxy \Dox"y\, n.; pl. Doxies. [See Duck a pet.]
   A loose wench; a disreputable sweetheart. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

doxy
    n 1: a woman who cohabits with an important man [syn:
         concubine, courtesan, doxy, paramour]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "doxy":
   chippy, concubine, floozy, girl friend, grisette, inamorata,
   kept mistress, kept woman, lover, mistress, nymph, nymphet,
   paramour, playmate, tart, unofficial wife, wanton, woman