means A person who secretly gathers information about an enemy, rival, or government, usually by infiltration and deception.

from From Old French 'espier,' to watch or observe, which English borrowed in the Middle Ages. That French word traces back to a Germanic root — the same family that gives us 'espy' and the modern 'spy' alike — built on an ancient Indo-European notion of looking, watching, peering. So at its core the word is simply about the act of seeing what others would rather keep hidden; the cloak and dagger came later.