The Collaborative International Dictionary of English v.0.48:

Spy \Spy\, n.; pl. Spies (sp[imac]z). [See Spy, v., and cf.
   Espy, n.]
   1. One who keeps a constant watch of the conduct of others.
      "These wretched spies of wit." --Dryden.
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   2. (Mil.) A person sent secretly into an enemy's camp,
      territory, or fortifications, to inspect his works,
      ascertain his strength, movements, or designs, and to
      communicate such intelligence to the proper officer.
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   Spy money, money paid to a spy; the reward for private or
      secret intelligence regarding the enemy.

   Spy Wednesday (Eccl.), the Wednesday immediately preceding
      the festival of Easter; -- so called in allusion to the
      betrayal of Christ by Judas Iscariot.
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   Syn: See Emissary, and Scout.
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