Stupidity is a lack of intelligence, understanding, reason, wit or sense. Stupidity may be innate, assumed or reactive – a defence against grief or trauma.
The root word stupid, which can serve as an adjective or noun, comes from the Latin verb stupere, for being numb or astonished, and is related to stupor. In Roman culture, the stupidus was the professional fall-guy in the theatrical mimes.
According to the online Merriam-Webster dictionary, the words stupid and stupidity entered the English language in 1541. Since then, stupidity has taken place along with fool, idiot, dumb, moron, and related concepts as a pejorative appellation for human misdeeds, whether purposeful or accidental, due to absence of mental capacity. (source : wikipedia)