Etymology
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dotard (n.)

late 14c., "imbecile, one who is in dotage or second childhood;" see dote (v.) + -ard. Sense of "one who dotes, one who is foolishly fond" (c. 1600) is now rare or obsolete. Other noun derivatives of dote, all in the sense "fool, simpleton" in Middle English were dotel (late 14c.), doterel (late 15c.), doti-poll (c. 1400; see doddypoll).

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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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doddypoll (n.)

also dotipoll, c. 1400, dotypolle, dodipoll, "stupid person," now obsolete in whatever spelling. The second element is poll (n.) in the original sense of "head." The first element is probably from Middle English dote (n.) "fool, simpleton, senile old man" (mid-12c.), from dote (v.). But it is sometimes said to be from Middle English dodden "to shear, shave."

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putz (n.)

"obnoxious man, fool," by 1964, from Yiddish, from German putz, literally "finery, adornment," obviously used here in an ironic sense. Attested in writing earlier in slang sense of "penis" (1934, in "Tropic of Cancer"). A non-ironic sense is in putz "Nativity display around a Christmas tree" (1873), from Pennsylvania Dutch (German), which retains the old German sense.

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motley (adj.)

late 14c., "parti-colored, variegated in color" (originally of fabric), from Anglo-French motteley, a word of unknown origin, perhaps [OED] based on Old English mot "speck" or a cognate Germanic word (see mote). But Klein's sources say probably from Gaulish. Century Dictionary rejects both. "Diversified in color," especially of a fool's dress. Hence, allusively, "a fool" (1600). As a noun meaning "cloth of contrasting mixed color" from late 14c.

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moppet (n.)

endearing term for a baby, a girl, etc., c. 1600," also "puppet made of cloth, rag-baby" (Johnson, 1755), from Middle English moppe "little child, baby doll" (mid-15c.) + -et, diminutive suffix. The Middle English word also meant "simpleton, fool," and may have been cognate with Low German mop "simpleton" [Barnhart]. Or, if "baby doll" is the original sense in Middle English, perhaps it is from Latin mappa "napkin, tablecloth," hence "rag doll."

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marmoset (n.)

late 14c., "small ape or monkey," from Old French marmoset "grotesque figurine; fool, jester" (late 13c.), perhaps a variant of marmote "long-tailed monkey, ape," then, as a term of endearment, "little child." It is said to be from marmonner, marmotter "to mutter, mumble," which is probably of imitative origin. Some French authorities suggest a derivation of marmoset from marmor "marble," as if "little marble figurine." The English word was applied from early 17c. specifically to a type of small squirrel-like South American monkey.

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gooseberry (n.)
type of thorny shrub with hairy fruit, cultivated in northern Europe, 1530s, with berry, but the first part is of uncertain origin; no part of the plant seems to suggest a goose. Watkins points to Old French grosele "gooseberry," which is from Germanic. Or perhaps from German Krausebeere or Kräuselbeere, related to Middle Dutch croesel "gooseberry," and to German kraus "crispy, curly" [Klein, etc.]. By either path it could be related to the Germanic group of words in kr-/cr- and meaning "to bend, curl; bent, crooked; rounded mass." Under this theory, gooseberry would be folk etymology. But OED editors find no reason to prefer this to a literal reading, because "the grounds on which plants and fruits have received names associating them with animals are so commonly inexplicable, that the want of appropriateness in the meaning affords no sufficient ground for assuming that the word is an etymological corruption."

As slang for a fool, 1719, perhaps an extended form of goose (n.) in this sense, or a play on gooseberry fool in the cookery sense. Gooseberry also meant "a chaperon" (1837) and "a marvelous tale." Old Gooseberry for "the Devil" is recorded from 1796. In euphemistic explanations of reproduction to children, babies sometimes were said to be found under a gooseberry bush.
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infatuate (v.)

1530s, "turn (something) to foolishness, frustrate by making foolish," from Latin infatuatus, past participle of infatuare "make a fool of," from in- "in" (from PIE root *en "in") + fatuus "foolish" (see fatuous). Specific sense of "inspire (in someone) a foolish passion beyond control of reason" is from 1620s. Related: Infatuated; infatuating.

An infatuated person is so possessed by a misleading idea or passion that his thoughts and conduct are controlled by it and turned into folly. [Century Dictionary]
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nincompoop (n.)

1670s, nicompoop; modern form from 1713. Despite similarity [noted by Johnson] to Latin legal phrase non compos mentis "insane, mentally incompetent" (c. 1600), the connection is denied by the OED's etymologists because the earliest forms lack the second -n-. Weekley thinks first element may be a proper name, and cites Nicodemus, which he says was used in French for "a fool," or Nicholas. Klein says it is probably an invented word. Century Dictionary has no objection to the non compos mentis theory.

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