be on the watch

be on the watch (for someone or something)

To be closely monitoring for someone or something, usually in an attempt to avoid danger. You need to be on the watch for suspicious people when you're out in the city this late at night. Be on the watch for grammar errors—this writer is notorious for run-on sentences.
See also: on, someone, watch
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

be on the ˈwatch (for somebody/something)

be looking carefully for somebody/something that you expect to see, especially in order to avoid possible danger: The police warned tourists to be on the watch for car thieves.
See also: on, watch
Farlex Partner Idioms Dictionary © Farlex 2017
See also:
References in classic literature ?
He was afraid to try to go through his companion's pockets, however; and besides the cabbie might be on the watch. He had the hundred safe, and he would have to be content with that.
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