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late 13c. (late 12c. as a surname), from Old French mostarde "mustard, mustard plant" (Modern French moutarde), from moust "must," from Latin mustum "new wine" (see must (n.1)); so called because it was originally prepared by adding must to the ground seeds of the plant to make a paste. As a color name, it is attested from 1848.
Mustard gas, World War I poison (first used by the Germans at Ypres, 1917), so called for its color and smell and burning effect on eyes and lungs; chemical name is dichlordiethyl sulfide, it contains no mustard, and is an atomized liquid, not a gas. To cut the mustard (1907, usually in negative) is probably from slang mustard "genuine article, best thing" (1903) on notion of "that which enhances flavor."
I'm not headlined in the bills, but I'm the mustard in the salad dressing just the same. [O.Henry, "Cabbages and Kings," 1904]
a plant of the genus sinapis, a pod-bearing, shrub-like plant, growing wild, and also cultivated in gardens. The little round seeds were an emblem of any small insignificant object. It is not mentioned in the Old Testament; and in each of the three instances of its occurrence in the New Testament (Matt. 13:31, 32; Mark 4:31, 32; Luke 13:18, 19) it is spoken of only with reference to the smallness of its seed. The common mustard of Palestine is the Sinapis nigra. This garden herb sometimes grows to a considerable height, so as to be spoken of as "a tree" as compared with garden herbs.