verb intrans
(often + for) to work for a government or other organization by secretly gathering data about enemies or rivals; to serve as a spy
When did he start spying for the Russians?
(+ on) to watch somebody secretly
You were spying on me!
to catch sight of or spot (somebody or something)
At length I spied her in the crowd
(+ out) to find (something) out by spying or observation
You naughty, sneaking, wicked boy! I'll teach you to spy out my secrets … and to make fun of me! — Frank L Baum
spy out the land
to investigate how things stand before making any definite move
[Middle English spien from Old French espier, of Germanic origin]