Dictionary.net

• Literally, who lives? who goes there? the challenge of a sentinel; the phrase often used as an English noun. To be on the qui vive, to be on the lookout, as a sentinel; to be wide awake, eager, watchful, expectant.

1919 - The Concise Standard Dictionary of the English Language
By James Champlin Fernald

• On the q. v., on the alert, watching for something to happen. [French]

1919 - The concise Oxford dictionary of current English
By Sir Augustus Henry