Etymology
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idiot (n.)
Origin and meaning of idiot
early 14c., "person so mentally deficient as to be incapable of ordinary reasoning;" also in Middle English "simple man, uneducated person, layman" (late 14c.), from Old French idiote "uneducated or ignorant person" (12c.), from Latin idiota "ordinary person, layman; outsider," in Late Latin "uneducated or ignorant person," from Greek idiotes "layman, person lacking professional skill" (opposed to writer, soldier, skilled workman), literally "private person" (as opposed to one taking part in public affairs), used patronizingly for "ignorant person," from idios "one's own" (see idiom).

In plural, the Greek word could mean "one's own countrymen." In old English law, one who has been without reasoning or understanding from birth, as distinguished from a lunatic, who became that way. Idiot box "television set" is from 1959; idiot light "dashboard warning signal" is attested from 1961. Idiot savant attested by 1870.
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idiotic (adj.)
1713, from idiot + -ic or from Late Latin idioticus "uneducated, ignorant," in classical Latin, "of an ordinary person," from Greek idiotikos "unprofessional, unskilled; not done by rules of art, unprofessional," from idiotes "unskilled person" (see idiot). Idiotical is from 1640s. Related: Idiotically.
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idiocy (n.)
"state of being an idiot," 1520s, from idiot on model of prophecy, etc. Early alternatives included idiotacy (1580s), idiotry (1590s).
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schmoe (n.)
also schmo, "idiot, fool," 1948, euphemized form of schmuck.
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blithering (adj.)
1880, present-participle adjective (from the first typically with idiot) from blither (v.) "to talk nonsense." From 1872 as a verbal noun.
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fuckwit (n.)
"fool idiot," slang, c. 1970, originally British or Australian English, from fuck + wit (n.).
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ament (n.)
"person born an idiot," 1894, from Latin amentia "madness," from amentem "mad," from a for ab "away from" (see a- (2)) + mentem "mind," from PIE root *men- (1) "to think."
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doofus (n.)
student slang, "dolt, idiot, nerd," by 1960s. "Dictionary of American Slang" says "probably related to doo-doo and goofus."
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moon-calf (n.)

also mooncalf, "abortive, shapeless, fleshy mass," 1560s, attributed to the influence of the moon; from moon (n.) + calf (n.1). In later 16c., "deformed creature, monster;" from 1620s as "congenital idiot."

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cockade (n.)

"clasp, button, etc. used to secure the cock of a hat," hence "any knot or badge worn on a hat," especially as a sign of political adherence, 1709, earlier cockard (1650s), from French cocarde (16c.), fem. of cocard (Old French cocart) "foolishly proud, cocky," as a noun, "idiot, fool;" an allusive extension from coq (see cock (n.1)).

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