late 13c. (late 12c. as a surname), "seed of the mustard plant crushed and used as a condiment paste or for medicinal purposes," 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 the name of the plant itself, by mid-14c. in English. As a color name, it is attested from 1848.
Mustard-pot is attested from early 15c. 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]