Etymology
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filet (n.)

1841 in cookery, reborrowing from French of the same word that had been taken 14c. and Englished as fillet (q.v.). Filet mignon (literally "dainty fillet") for "small, round, tender cut of meat from the center of the fillet" is attested as a French word in English from 1815.

The 'Chateaubriand,' the 'entrecôte,' and the 'filet mignon' (of mutton), with other forms, are all due to the more enlarged sympathies of the French butcher for what is perfect. We must entirely change the mode of cutting up the carcase before we can arrive at the same perfection in form of meat purchasable, and as that is hopeless, so is it useless to insist further on the subject on behalf of the public. ["The Kitchen and the Cellar," Quarterly Review, April 1877]
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fillet (n.)
early 14c., "little headband," from Old French filet "thread, filament; strip, ligament" (12c.), diminutive of fil "thread" (see file (v.1)). Sense of "cut of meat or fish" is from late 14c., apparently so called because it was prepared by being tied up with a string.
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