WordNet (r) 3.0 (2006):

sitting duck
    n 1: a defenseless victim [syn: sitting duck, easy mark]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "sitting duck":
   babe, boob, breeze, butt, chump, cinch, credulous person, cull,
   dupe, easy mark, easy pickings, easy target, easy thing, fall guy,
   fish, fool, gobe-mouches, greener, greenhorn, greeny, gudgeon,
   gull, innocent, leadpipe cinch, mark, monkey, patsy, picnic, pie,
   piece of cake, pigeon, plaything, prize sap, pushover, sap,
   saphead, schlemiel, setup, sinecure, sitter, snap, stooge, sucker,
   toy, trusting soul, velvet, victim