Definition of spy on
: to watch (someone) secretly He spies on his neighbors. Have you been spying on me?
Word by Word Definitions
: to watch secretly usually for hostile purposes
: to catch sight of : see
: to observe or search for something : look
: one that spies:
: one who keeps secret watch on a person or thing to obtain information
: a person employed by one nation to secretly convey classified information of strategic importance to another nation
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