Source : Webster's Revised Unabridged Dictionary (1913)

Donkey \Don"key\, n.; pl. {Donkeys}. [Prob. dun, in allusion to
   the color of the animal + a dim. termination.]
   1. An ass; or (less frequently) a mule.

   2. A stupid or obstinate fellow; an ass.

   {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.

Source : WordNet®

donkey engine
     n 1: a locomotive for switching rolling stock in a railroad yard
          [syn: {switch engine}]
     2: (nautical) a small engine (as one used on board ships to
        operate a windlass) [syn: {auxiliary engine}]