upon one's guard

Thesaurus

Adj.

1.

upon one's guard - vigilant; "on guard against con artists"; "must remain on your guard in such a situation" on guard, on one's guard, on your guard wary - marked by keen caution and watchful prudence; "they were wary in their movements"; "a wary glance at the black clouds"; "taught to be wary of strangers"

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