retard (v.)

late 15c., "make slow or slower," from French retarder "restrain, hold (someone) back, keep (someone from doing something); come to a stop" (13c.) or directly from Latin retardare "make slow, delay, keep back, hinder" (see retardation). Related: Retarded; retarding.

The noun is recorded from 1788 in the sense "retardation, delay;" from 1970 in offensive meaning "retarded person," originally American English, with accent on first syllable. Other words used for "one who is mentally retarded" include retardate (1956), retardee (1971).

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Definitions of retard from WordNet
retard (v.)
cause to move more slowly or operate at a slower rate;
This drug will retard your heart rate
retard (v.)
be delayed;
retard (v.)
slow the growth or development of;
The brain damage will retard the child's language development
Synonyms: check / delay
retard (v.)
lose velocity; move more slowly;
Synonyms: decelerate / slow / slow down / slow up
retard (n.)
a person of subnormal intelligence;