a (context|psychology|now|_|offensive) Having mentalretardation, characterised by learning difficulties (specifically having an IQ below 70); now more generally, mentally deficient. (defdate|from 19th c.)
a (context|chiefly|North America)underdeveloped | Underdeveloped (of a person's intelligence, abilities etc.). (defdate|from 20th c.)
a (North America|colloquial|derogatory)stupid | Stupid, dumb. (defdate|from 20th c.)
a (physics) Designating a parameter of an electromagnetic field which is adjusted to account for the finite speed of radiation. (defdate|from 20th c.)