qui vive

1726, in on the qui vive "on the alert," from French qui voulez-vous qui vive? sentinel's challenge, "whom do you wish to live," literally "(long) live who?" In other words, "whose side are you on?" (The answer might be Vive la France, Vive le roi, etc.).

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Definitions of qui vive from WordNet

qui vive (n.)
condition of heightened watchfulness or preparation for action;
Synonyms: alert