simple past and past participle of retard

Delayed; delayed in development, hindered; impeded.

(psychology, sometimes offensive, dated) Having mental retardation; mentally deficient or underdeveloped.

(psychology, sometimes offensive, dated) Specifically, having an IQ below 70.

(colloquial, derogatory, offensive) Extremely stupid.

(physics) Designating a parameter of an electromagnetic field which is adjusted to account for the finite speed of radiation.