Wiki article on this topic:
Stupidity is a lack of intelligence, understanding, reason, wit, or common 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 for misdeeds, whether purposeful or accidental, due to absence of mental capacity.
Stupidity is a quality or state of being stupid, or an act or idea that exhibits properties of being stupid. In a character study of "The Stupid Man" attributed to the Greek philosopher Theophrastus , stupidity was defined as "mental slowness in speech or action". The modern English word "stupid" has a broad range of application, from being slow of mind , dullness of feeling or sensation , or lacking interest or point. It can either imply a congenital lack of capacity for reasoning, or a temporary state of daze, or slow-mindedness.
In Understanding Stupidity, James F. Welles defines stupidity this way: "The term may be used to designate a mentality which is considered to be informed, deliberate and maladaptive." Welles distinguishes stupidity from ignorance; one must know they are acting in their own worst interest. Secondly, it must be a choice, not a forced act or accident. Lastly, it requires the activity to be maladaptive, in that it is in the worst interest of the actor, and specifically done to prevent adaption to new data or existing circumstances."
Carlo Maria Cipolla, an economic historian, is famous for his essays about human stupidity, such as "The Basic Laws of Human Stupidity". He viewed stupid people as a group, more powerful by far than major organizations such as the Mafia and the industrial complex, which without regulations, leaders or manifesto nonetheless manages to operate to great effect and with incredible coordination.