When a sentinel guarding a French castle in days of yore cried, Qui vive? your life depended upon your answer - the right one was usually something like "Long live the king!" The question the sentinel was asking was "Long live who?" but the act of calling out apparently impressed English listeners more than the meaning of the phrase, because when they adopted it in the early 1700s they used qui vive to mean "alert." Nowadays, the term is most often used in the phrase on the qui vive, meaning "on the lookout."