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spy on (someone or something)
To observe someone or something in a clandestine or furtive manner, so as not to be noticed doing so. I can't believe you hired a private detective to spy on me—do you really distrust me that much? I was sent to spy on the foreign ambassador during his time in Washington.
spy (up)on someone or something
to watch someone or something to learn secret or concealed information. (Upon is formal and less commonly used than on.) Are you spying upon me? I wasn't spying on you! I was just trying to see who you were.
Secretly or furtively observe someone or something, as in The children loved spying on the grownups, or The company sent him to spy on the competitor's sales force. [Early 1600s]
spy onor spy upon
To observe someone or something secretly and closely: A detective had been spying on the mayor's every move for months. The children spied upon their neighbors from the bushes along the property line.