This post lists words and phrases used in English that are derived from French but are no longer employed with the same idiomatic sense in French (if they ever were).
Each term is followed by the literal French translation, a brief definition, and a comment about its status in French and how the French language conveys the idiom.
1. au jus (“with juice”): a brothlike meat sauce (the phrase is often incorrectly treated on menus as “with au jus”)—obsolete in French except for the slang phrase être au jus (roughly, “be with juice”)
2. cause célèbre (“celebrated cause”): controversial or emotionally weighted issue—obsolete in French, but originally referred to a sensational or unusual legal decision or trial
3. demimonde (“half world”): fringe group or subculture, or prostitutes as a class—obsolete in French, thoughune demi-mondaine refers to a prostitute (in English, demimondaine is synonymous with “kept woman”)
4. double entendre (“double to hear”): a comment that can have a second, often provocative, connotation—faulty grammar in French, which uses à double sens (“double sense”)