qui vive

1726, in on the qui vive "on the alert," from French être sur le qui vive "be on the alert," from the phrase qui voulez-vous qui vive? sentinel's challenge, "whom do you wish to live?" In other words "(long) live who?" meaning "whose side are you on?" (The answer might be Vive la France, Vive le roi, etc.). From qui (from Latin qui "who") + vive, third person singular present subjunctive of vivre, from Latin vivere "to live" (see viva).

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Definitions of qui vive from WordNet

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