A pseudo-French expression in English is a word or expression in English that has the appearance of having been borrowed from French, but which in fact was created in English and does not exist in French.
Several such French expressions have found a home in English. The first continued in its adopted language in its original obsolete form centuries after it had changed its form in national French:
bon viveur — the second word is not used in French as such, while in English it often takes the place of a fashionable man, a sophisticate, a man used to elegant ways, a man-about-town, in fact a bon vivant
double entendre — French speaker would use double sens to render this; double entendre has no meaning in French.
legerdemain (supposedly from, léger de main, literally, "light of hand": sleight of hand, usually in the context of deception or the art of stage magic tricks. Meaningless in French, and has no equivalent.
nom de plume — coined in the 19th century in English, on the pattern of nom de guerre, which is an actual French expression, where "nom de plume" is not.
voir dire — a legal phrase for a variety of procedures connected with jury trials. Originally from Anglo-Norman, means nothing in France, but has been picked up in Canadian legal French with the same meaning.