amentia

n. lack of intellectual development, imbecility, idiocy (Psychiatry)

Amentia /amen·tia/ former term for profound mental retardation. a·men·tia n. Insufficient mental development or functioning. [more]Amentia - Community and Resources

Noun
1. extreme mental retardation
(synonym) idiocy
(hypernym) retardation, mental retardation, backwardness, slowness, subnormality

(n.)
Imbecility; total want of understanding.
  

noun
being mentally subnormal