From 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.
[1913 Webster]
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.
[1913 Webster]
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.
[1913 Webster]