au fait (o FAY)
Being well-informed or skillful in something. An expert.
Literally "to the fact".
hors d'oeuvre (ohr DERV)
An appetizer or an extra little dish outside of and smaller than the main course. It is usually served before or as the first course of a meal.
hors (outside of) + oeuvre (job or work).
carte blanche (kart blansh)
carte blanche (blank card or blank document).
qui vive (kee VEEV)
Alert, lookout. (It is usually used in the form "on the qui vive").
Literally "(Long) live who?"
It was used by soldiers at the entrance to forts etc. to challenge someone approaching the gate. A proper response might be "Vive le roi!" (Long Live the King) or "La France!"
mot juste (moh zhyst)
The exact, appropriate word.
mot (word) + juste (right).