Etymology
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must (n.3)

"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)).

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hop (v.1)
Old English hoppian "to spring, leap; to dance; to limp," from Proto-Germanic *hupnojan (source also of Old Norse hoppa "hop, skip," Dutch huppen, German hüpfen "to hop"). Transitive sense from 1791. Related: Hopped; hopping. Hopping-john "stew of bacon with rice and peas" attested from 1838. Hopping mad is from 1670s.
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math (n.2)

"a mowing, what is gathered from mowing," Old English mæð "mowing, cutting of grass," from Proto-Germanic *mediz (source also of Old Frisian meth, Old High German mad, German Mahd "mowing, hay crop"), from PIE root *me- (4) "to cut down grass or grain." Obsolete except in figurative aftermath.

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psychiatrist (n.)

"one who practices psychiatry," 1875, from psychiatry + -ist.

A psychiatrist is a man who goes to the Folies Bergère and looks at the audience. [Anglican Bishop Mervyn Stockwood, 1961]

An older name was mad-doctor (1703); also psychiater "expert in mental diseases" (1852), from Greek psykhē + iatros. Also see alienist.

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insanity (n.)
1580s, "state of being insane, seriously impaired state of mental functioning," from Latin insanitatem (nominative insanitas) "unhealthfulness, unsoundness, disease," noun of quality from insanus "mad, insane; outrageous, excessive" (see insane). Meaning "extreme folly" is from 1844. The Latin abstract noun was insania ""unsoundness of mind, madness, frenzy."
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Caligula 
cognomen of the mad, extravagant, and legendarily cruel third Roman emperor (12 C.E.-41 C.E.), born Gaius Caesar. The nickname is Latin, literally "little boot," given when he joined his father on military campaigns when still a toddler, in full, child-sized military gear; it is a diminutive of caliga "heavy military shoe," which is of unknown origin.
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muesli (n.)

breakfast dish of oats, fruit, and nuts, eaten with milk or yogurt, 1926, from Swiss-German, from Old High German muos "meal, mush-like food," from Proto-Germanic *mod-sa-, from PIE root *mad- "moist, wet," with derivatives referring to various qualities of food (see mast (n.2)).

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loco (adj.)
"mad, crazy," 1844, American English, from Spanish loco (adj.) "insane," of uncertain origin, perhaps from Arabic lauqa, fem. of 'alwaq "fool, crazy person." Loco-weed was the name given to species of western U.S. plants that cause cattle and horse diseases that make them stagger and act strangely. But the adjective seems to be the older word.
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redwood (n.)

also red-wood, 1610s, "wood that has a red hue," from red (adj.1) + wood (n.). Of various types of New World trees that yield such wood, from 1716; specifically the California Sequoia sempervirens from 1819. In Scottish English 16c.-18c. the same word as an adjective meant "completely deranged, raving, stark mad," from wood (adj.).

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funk (n.1)
"depression, ill-humor," perhaps from earlier sense "cowering state of fear" (1743), identified in OED as originally Oxford slang, probably from Scottish and Northern English verb funk "become afraid, shrink through fear, fail through panic," (1737), of unknown origin. Perhaps from Flemish fonck "perturbation, agitation, distress," which is possibly related to Old French funicle "wild, mad."
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