The Collaborative International Dictionary of English v.0.48:

doctor \doc"tor\, n. [OF. doctur, L. doctor, teacher, fr. docere
   to teach. See Docile.]
   1. A teacher; one skilled in a profession, or branch of
      knowledge; a learned man. [Obs.]
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            One of the doctors of Italy, Nicholas Macciavel. --
                                                  Bacon.
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   2. An academical title, originally meaning a man so well
      versed in his department as to be qualified to teach it.
      Hence: One who has taken the highest degree conferred by a
      university or college, or has received a diploma of the
      highest degree; as, a doctor of divinity, of law, of
      medicine, of music, or of philosophy. Such diplomas may
      confer an honorary title only.
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   3. One duly licensed to practice medicine; a member of the
      medical profession; a physician.
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            By medicine life may be prolonged, yet death
            Will seize the doctor too.            -- Shak.
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   4. Any mechanical contrivance intended to remedy a difficulty
      or serve some purpose in an exigency; as, the doctor of a
      calico-printing machine, which is a knife to remove
      superfluous coloring matter; the doctor, or auxiliary
      engine, called also donkey engine.
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   5. (Zool.) The friar skate. [Prov. Eng.]
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   Doctors' Commons. See under Commons.

   Doctor's stuff, physic, medicine. --G. Eliot.

   Doctor fish (Zool.), any fish of the genus Acanthurus;
      the surgeon fish; -- so called from a sharp lancetlike
      spine on each side of the tail. Also called barber fish.
      See Surgeon fish.
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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.
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   2. A stupid or obstinate fellow; an ass.
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   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.
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WordNet (r) 3.0 (2006):

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