be on the watch

be on the watch (for someone or something)

To look out 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.
See also: on, someone, watch

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
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.
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