spy on

spy on idiom
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]

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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