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"a witty saying," 1580s, from French mot (12c.) "remark, short speech," literally "word," cognate of Italian motto, from Latin mutum "grunt, murmur" (see mutter). Mot juste (1912) is French, literally "exact word," the precisely appropriate expression in some situation.
The mot juste is an expression which readers would like to buy of writers who use it, as one buys one's neighbour's bantam cock for the sake of hearing its voice no more. [Fowler]