1190, from O.Fr.
moustarde, from
moust "must," from L.
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]
[muhs-terd] 
tərd