"mold, moldiness," c. 1600, perhaps a back-formation of musty (q.v.).

auxiliary of prediction, "be obliged, be necessarily impelled," from Old English moste, past tense of motan "have to, be able to," from Proto-Germanic *motanan (source also of Old Saxon motan "to be obliged to, have to," Old Frisian mota, Middle Low German moten, Dutch moeten, German müssen "to be obliged to," Gothic gamotan "to have room to, to be able to"), perhaps from PIE root *med- "take appropriate measures," but this old suggestion lately has been doubted. Used as present tense from c. 1300, eventually displacing motan, from the custom of using past subjunctive as a moderate or polite form of the present.

"male elephant frenzy," 1878, from earlier adjective (1855), from Urdu mast "intoxicated, in rut," from Persian mast, literally "intoxicated," related to Sanskrit matta- "drunk, intoxicated," past participle of madati "boils, bubbles, gets drunk," from PIE root *mad- "wet, moist" (see mast (n.2)).

"that which has to be done, seen, or experienced," 1892, from must (v.). As an adjective, "obligatory, indispensable," by 1912, from the noun; must-read (n.) is from 1959.

also moustache (chiefly British), "the hair that grows upon the upper lip of men," 1580s, from French moustache (15c.), from Italian mostaccio, from Medieval Greek moustakion, diminutive of Doric mystax (genitive mystakos) "upper lip, mustache," related to mastax "jaws, mouth," literally "that with which one chews," perhaps from PIE root *mendh- "to chew" (see mandible), but Beekes says this whole group of Greek words may be of Pre-Greek origin.

Borrowed earlier (1550s) as mostacchi, from the Italian word or its Spanish derivative mostacho. The plural form of this, mustachios, lingers in English (the mustache sometimes was considered as the hair on either side of the lip, hence the use of the plural form). Slang shortening stache attested from 1985. Old English had cenep "mustache," which is related to cnafa "boy" (see knave). Mustache-cup, one with a fixed cover over part of its top, allowing one to drink without dipping the mustache, is by 1868.

"wearing a mustache," 1817, from mustachio (1550s), from Spanish mostacho and directly from Italian mostaccio (see mustache). The noun was superseded by mustache, but the adjective has endured.

"small, half-wild horse of the American prairie and pampas," 1808, from Mexican Spanish mestengo "animal that strays" (16c.), from Spanish mestengo "wild, stray, ownerless," literally "belonging to the mesta," an association of cattle ranchers who divided stray or unclaimed animals that got "mixed" with the herds, from Latin mixta "mixed," fem. past participle of miscere "to mix" (from PIE root *meik- "to mix").

Said to be influenced by the Spanish word mostrenco "straying, wild," which is probably from mostrar, from Latin monstrare "to show." Though now feral, the animals are descended from tame horses brought to the Americas by the Spaniards. The brand of automobile was introduced by Ford in 1962.

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]

also mestee, "octoroon, offspring of a white and a quadroon," also, generally, "a half-caste," 1690s, a West Indian word, a corruption of Spanish mestizo (q.v.).

"animal of the family of mammals that includes the weasels, badgers, skunks, and otters," 1910, from Modern Latin Mustelidae, taken as a genus name by Linnaeus (1758), from Latin mustela "weasel," which is possibly a diminutive form from mus "mouse" (see mouse (n.)), a theory accepted by de Vaan, who writes, "The use of the dim. for the weasel can be due to its small size compared with other similar animals (marten, polecat) or because it was domesticated and used as a pet animal." Tucker tentatively suggests *mus-ters-la "mouse harrier," and Klein notes that the weasel was identified in antiquity as "the catcher of mice."