Etymology
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retardation (n.)

early 15c., retardacion, "fact or action of delaying or making slower in movement or time," from Latin retardationem (nominative retardatio) "a delaying," noun of action from past participle stem of retardare "to make slow, delay, keep back, hinder," from re- "back" (see re-) + tardare "to slow," related to tardus "slow, sluggish" (see tardy).

The psychological sense of "educational slowness, educational progress slower than average for a group" is from 1907, perhaps a back-formation from retarded. For the meaning "act of retarding," retardment also was used (1640s).

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ritardando (adv.)

in music, "becoming gradually slower," 1811, from Italian, present participle of ritardare "to slow down," from Latin retardare "to make slow" (see retardation). The Italian plural is ritardandi.

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retard (v.)

late 15c., retarden, "make slow or slower; keep back, hinder, delay" (transitive), from French retarder "restrain, hold (someone) back, keep (someone from doing something); come to a stop" (13c.) and directly from Latin retardare "make slow, delay, keep back, hinder" (see retardation). Related: Retarded; retarding. The intransitive sense of "be delayed" is from 1640s. 

The noun retard is recorded from 1788 in the sense "retardation, delay;" from 1970 in the offensive meaning "retarded person," originally American English, with accent on first syllable. Other words used for "one who is mentally retarded" include retardate (1956, from Latin retardatus), and U.S. newspapers 1950s-60s often used retardee (1950).

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Mongoloid 

1868, adj. and noun, as a racial designation, literally "resembling the Mongols," from Mongol + -oid. Compare Mongolian. In reference to the genetic defect causing mental retardation (mongolism), by 1899, from the typical facial appearance of those who have it. See Down's Syndrome. Such people were called Mongolian from 1866. 

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retardance (n.)

1550s, "slowness, a making slower, retardation," from French retardance, from retarder (see retard (v.)). It seems to persist in reference to resistance to fire, in which sense it dates from 1921. Related: Retardancy.

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lag (n.)

in the mechanical sense "retardation of movement," 1855, from lag (v.). Also noted in Farmer and Henley ("Slang and Its Analogues") as American theatrical slang for "a wait," with an attestation from 1847. First record of lag time is from 1951.

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retarded (adj.)

1810, "delayed," past-participle adjective from retard (v.). In childhood development psychology, "mentally slow, lagging significantly in mental or educational progress," especially if due to some impairment, attested from 1895 (G.E. Shuttleworth, "late medical superintendent, Royal Albert Asylum, for idiots and imbeciles of the northern counties, Lancaster," perhaps inspired by Italian tardivi). Its application has shifted over the years based on what the progress or lack of it was measured against (peers, a score on IQ tests, etc.), but the progress gap was deemed "significant."

Fashions in labeling this group change almost from year to year; in the 1960s, mental retardation was the favorite appellation, and justifiably so in that it does not imply that inheritance or constitutional defects are always the cause of mental retardation. ["Campbell's Psychiatric Dictionary," 2004]
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mental (adj.)

early 15c., "in, of, or pertaining to the mind; characteristic of the intellect," from Late Latin mentalis "of the mind," from Latin mens (genitive mentis) "mind," from PIE root *men- (1) "to think."

In Middle English, also "of the soul, spiritual." From 1520s as "done or performed in the mind." Meaning "crazy, deranged" is by 1927, probably from combinations such as mental patient (1859); mental hospital (1891). Mental health is attested by 1803; mental illness by 1819; mental retardation by 1904.

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