Qui Vive
Definition: on alert, or on the lookout
It’s a French phrase literally meaning “Long live who?” Guards and sentries would ask unknown people this question, looking for a specific answer (presumably “long live the king,” though one wonders how this didn’t simply become common knowledge).
The practical definition and usage eventually morphed into something like, “What side are you on?” or “Who goes there?”
Today, it’s virtually only used within the phrase “on the qui vive,” which means someone is suspicious or alertly looking for something.
Why I Looked It Up
From The Money Kings:
Detectives and agents provocateurs are always on the qui vive.