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 pump
    n 1: a supplementary pump available if needed [syn: auxiliary
         pump, donkey pump]