The Collaborative International Dictionary of English v.0.48:
Spy \Spy\, n.; pl. Spies (sp[imac]z). [See Spy, v., and cf.
1. One who keeps a constant watch of the conduct of others.
"These wretched spies of wit." --Dryden.
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.
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.
Syn: See Emissary, and Scout.