qui vive

When a sentinel guarding a French castle in days of yore cried, "Qui vive?," your life depended upon your answer. The question the sentinel was asking was "Long live who?" The correct answer was usually something like "Long live the king!" Visitors not answering the question this way were regarded as suspect, and so to be "on the qui vive" meant to be on the alert or lookout, and qui vive came to mean "alert" or "lookout" soon afterward. Nowadays, the term is most often used in the phrase "on the qui vive," meaning "on the lookout."