What does Boxing Day have to do with boxing?
1810, "delayed," past participle adjective from retard (v.). In childhood development sense, "mentally slow," attested from 1895 (cf. Italian tardivi).
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).
Defective and annoying: this retarded bicycle