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]