Source : Webster's Revised Unabridged Dictionary (1913)

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. (Zo["o]l.) A quadruped of the genus {Equus} ({E. 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.

   2. A dull, heavy, stupid fellow; a dolt. --Shak.

   {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.

Source : WordNet®

ass
     n 1: the fleshy part of the human body that you sit on; "he
          deserves a good kick in the butt"; "are you going to sit
          on your fanny and do nothing?" [syn: {buttocks}, {nates},
           {arse}, {butt}, {backside}, {bum}, {buns}, {can}, {fundament},
           {hindquarters}, {hind end}, {keister}, {posterior}, {prat},
           {rear}, {rear end}, {rump}, {stern}, {seat}, {tail}, {tail
          end}, {tooshie}, {tush}, {bottom}, {behind}, {derriere},
           {fanny}]
     2: a pompous fool
     3: hardy and sure-footed animal smaller and with longer ears
        than the horse
     4: slang terms for sexual intercourse [syn: {fuck}, {fucking},
        {screw}, {screwing}, {nooky}, {nookie}, {piece of ass}, {piece
        of tail}, {roll in the hay}, {shag}, {shtup}]