WordNet (r) 3.0 (2006):

patsy
    n 1: a person who is gullible and easy to take advantage of
         [syn: chump, fool, gull, mark, patsy, fall guy,
         sucker, soft touch, mug]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "patsy":
   babe, boob, chump, cinch, credulous person, cull, dupe, easy mark,
   easy pickings, fall guy, fish, fool, goat, gobe-mouches, greener,
   greenhorn, greeny, gudgeon, gull, innocent, leadpipe cinch, mark,
   monkey, pigeon, plaything, prize sap, pushover, sap, saphead,
   scapegoat, schlemiel, sitting duck, stooge, sucker, toy,
   trusting soul, victim, whipping boy