Qui vive! A sentinel's challenge.
Idiom: on the qui vive
On the alert; vigilant: “a loathsome Dublin politico who is on the qui vive for... terrorists” (Julian Moynahan).
[French, (long) live who? (a sentry's challenge to determine a person's political sympathies) : qui, who + vive, third person sing. present subjunctive of vivre, to live.]
qui vive used like the English challenge: ``Who comes there?''

To be on the qui vive, to be on guard; to be watchful and alert, like a sentinel.
qui vive n : condition of heightened watchfulness or preparation for action: "bombers were put on alert during the crisis" [syn: alert]