The Collaborative International Dictionary of English v.0.48:

Sheepish \Sheep"ish\, a.
   1. Of or pertaining to sheep. [Obs.]
      [1913 Webster]

   2. Like a sheep; bashful; over-modest; meanly or foolishly
      diffident; timorous to excess.
      [1913 Webster]

            Wanting change of company, he will, when he comes
            abroad, be a sheepish or conceited creature.
                                                  --Locke.
      [1913 Webster] -- Sheep"ish*ly, adv. --
      Sheep"ish*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sheepish
    adj 1: like or suggestive of a sheep in docility or stupidity or
           meekness or timidity [syn: sheeplike, sheepish]
    2: showing a sense of shame [syn: shamefaced, sheepish]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "sheepish":
   abject, amenable, apologetic, asinine, blushful, blushing, bovid,
   bovine, caprid, caprine, compliant, contrite, cowish, cowlike,
   deerlike, docile, embarrassed, equestrian, equine, flushed,
   goatish, goatlike, hircine, hoggish, hoofed, horsy, humble,
   humbled, manipulable, meek, melted, mulish, obedient, ovine,
   passive, penitent, penitential, penitentiary, piggish, pliable,
   red, repentant, ruddy, ruminant, shamefaced, sheeplike, softened,
   swinish, timid, touched, tractable, ungulate, withdrawn