qui vive
Concise Oxford English Dictionary © 2008 Oxford University Press:
qui vive /kiː ˈviːv/
▶noun (in phr. on the qui vive) on the alert or lookout.
– origin C16: from Fr., lit. ‘(long) live who?’, i.e. ‘on whose side are you?’, used as a sentry's challenge.
'qui vive' also found in these Oxford entries:

