Source : Webster's Revised Unabridged Dictionary (1913)

Qui vive \Qui` vive"\ [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?''

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