>On the alert or lookout.
>On being observant
Qui vive is a french expression that has come into existence in English since the late 16th century. In literal terms, it means (long) live who? i.e on whose side are you? ‘In former times, such a provocation was issued by a sentry to someone coming towards his post so as to make sure which side their loyalty was’.
>Their duty requires most of the soldiers to be on the qui vive at the Siachen glaciers
>What he might do the next do was uncertain. His uncertainty always kept her on the qui vive to avoid any mishap.