The Collaborative International Dictionary of English v.0.48:

Ass \Ass\, n. [OE. asse, AS. assa; akin to Icel. asni, W. asen,
   asyn, L. asinus, dim. aselus, Gr. ?; also to AS. esol, OHG.
   esil, G. esel, Goth. asilus, Dan. [ae]sel, Lith. asilas,
   Bohem. osel, Pol. osiel. The word is prob. of Semitic origin;
   cf. Heb. ath?n she ass. Cf. Ease.]
   1. (Zool.) A quadruped of the genus Equus (Equus asinus),
      smaller than the horse, and having a peculiarly harsh bray
      and long ears. The tame or domestic ass is patient, slow,
      and sure-footed, and has become the type of obstinacy and
      stupidity. There are several species of wild asses which
      are swift-footed.
      [1913 Webster]

   2. A dull, heavy, stupid fellow; a dolt. --Shak.
      [1913 Webster]

   Asses' Bridge. [L. pons asinorum.] The fifth proposition of
      the first book of Euclid, "The angles at the base of an
      isosceles triangle are equal to one another." [Sportive]
      "A schoolboy, stammering out his Asses' Bridge." --F.
      Harrison.

   To make an ass of one's self, to do or say something very
      foolish or absurd.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Equus asinus
    n 1: a wild ass of Africa [syn: African wild ass, Equus
         asinus]
    2: domestic beast of burden descended from the African wild ass;
       patient but stubborn [syn: domestic ass, donkey, Equus
       asinus]