retarder (n.)

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).