What does spy mean?
The definition of spy
Spy, v. t.
[imp. & p. p. Spied; p. pr. & vb. n. Spying.] Etym: [OE. spien, espien, OF. espier, F. épier, OHG. speh, G. spähen; akin to L. specere to see, Skr. spa(Espy, v.t., Aspect, Auspice, Circumspect, Conspicuouc, Despise, Frontispiece, Inspect, Prospect, Respite, Scope, Scecimen, Spectacle, Specter, Speculate, Spice, Spite, Suspicion.]

To gain sight of; to discover at a distance, or in a state of concealment; to espy; to see. One in reading, skipped over all sentences where he spied a note of admiration. Swift.
2 To discover by close search or examination. Look about with yout eyes; spy what things are to be reformed in the church of England. Latimer.
3 To explore; to view; inspect; and examine secretly, as a country; — usually with out. Moses sent to spy Jaazer, and they took the villages thereof. Num. xxi. 32.

Spy, v. i.
To search narrowly; to scrutinize. It is my nature’s plague To spy into abuses. Shak.
Spy, n.; pl. Spies.
Etym: [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.
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.
Synonyms
— See Emissary, and Scout.