What is Stupidity?

Legal Definition
Stupidity is a lack of intelligence, understanding, reason, wit or sense. Stupidity may be innate, assumed or reactive – a defence against grief or trauma.
-- Wikipedia
Legal Definition
Med. jur. That state of the mind which cannot perceive and embrace the data presented to it by the senses; and therefore the stupid person can, in general, form no correct judgment. It is a want of the perceptive powers. Ray, Med. Jur. c. 3, §40. Vide Imbecility.
-- Bouviers Law Dictionary