[French: mot, word + juste, right]
--The American Heritage Dictionary of the English Language, 2000.
From Tad Tuleja in Foreignisms, pg.84:
Those who "always know just what to say" have an ample store of mot justes: a mot juste is the perfectly appropriate word (or phrase) for a given occasion. It may be a bon mot,* but it need not be; a bon mot at a funeral would probably not be funny. Useful as a disguise in those moments when something is on the tip of your tongue; it's less cliched to say "The mot juste escapes me"--with an air of cavalier insouciance**, of course--than to admit your bewilderment.
"If she couldn't make you think, at least she would make you laugh: she had a bon mot ready for every occasion." Don't confuse this with mot juste.--Tad Tuleja in Foreignisms, pg. 26.
Date "insouciance" was first used: 1799.
--WordNet 1.7.1 Copyright © 2001 by Princeton University. All rights reserved.
Q: What's the difference between yesterday's word riposte and today's mot juste?
A: The difference lies in circumstances. If during a round of badinage with a friend, you happily one-up your friend with "a sharp, witty response," you have delivered a riposte. If, on the other hand, while you are writing or speaking in a non-competitive setting, you come up with what you consider to be "exactly the right word or expression," you've just birthed a mot juste.