The Collaborative International Dictionary of English v.0.48:

Donkey \Don"key\ (d[o^][ng]"k[y^]), n.; pl. Donkeys
   (d[o^][ng]"k[i^]z). [Prob. dun, in allusion to the color of
   the animal + a dim. termination.]
   1. An ass; or (less frequently) a mule.
      [1913 Webster]

   2. A stupid or obstinate fellow; an ass.
      [1913 Webster]

   Donkey engine, a small auxiliary engine not used for
      propelling, but for pumping water into the boilers,
      raising heavy weights, and like purposes.

   Donkey pump, a steam pump for feeding boilers,
      extinguishing fire, etc.; -- usually an auxiliary.

   Donkey's eye (Bot.), the large round seed of the Mucuna
      pruriens, a tropical leguminous plant.
      [1913 Webster]

WordNet (r) 3.0 (2006):

donkey
    n 1: the symbol of the Democratic Party; introduced in cartoons
         by Thomas Nast in 1874
    2: domestic beast of burden descended from the African wild ass;
       patient but stubborn [syn: domestic ass, donkey, Equus
       asinus]

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "donkey":
   Boeotian, Rocky Mountain canary, ass, bigot, bitter-ender, block,
   bullethead, burro, clod, cuddy, dickey, diehard, dimwit, dogmatist,
   dolt, dope, dullard, dumb cluck, dumbbell, dummy, dunce, fanatic,
   gowk, hardnose, intransigeant, intransigent, jack, jackass, jennet,
   jenny, jenny ass, jobbernowl, lackwit, lamebrain, last-ditcher,
   lightweight, looby, loon, maverick, mule, neddy, niais, nincompoop,
   ninny, ninnyhammer, nitwit, noddy, perverse fool, pighead,
   positivist, purist, put, rocky, schmuck, standpat, standpatter,
   stickler, stupid, thickwit, witling