qui vive
on the qui vive on the alert or lookout.
☞ The French expression qui vive? (used in English since the late 16th century) means literally '(long) live who?' In former times a sentry would issue this challenge to someone approaching his post so as to ascertain where their allegiance lay.
1976J. E. WeemsDeath Song They came in groups of four, five, or six-'all on the qui vive, apprehensive of treachery, and ready to meet it'.