To be on the qui vive

from The Collaborative International Dictionary of English v.0.48

Qui vive \Qui` vive"\ (k[-e]` v[-e]v"). [F., fr. qui who + vive,
   pres. subj. of vivre to live.]
   The challenge of a French sentinel, or patrol; -- used like
   the English challenge: "Who comes there?"
   [1913 Webster]

   {To be on the qui vive}, to be on guard; to be watchful and
      alert, like a sentinel.
      [1913 Webster]