Stupidity is a lack of intelligence, understanding, reason, or wit, an inability to learn. It may be innate, assumed or reactive. The word stupid comes from the Latin word stupere. Stupid characters are often used for comedy in fictional stories. Walter B. Pitkin called stupidity "evil", but in a more Romantic spirit William Blake and Carl Jung believed stupidity can be the mother of wisdom.

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 (c. 371 – c. 287 BC), 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 (indicating a lack of intelligence, care or reason), dullness of feeling or sensation (torpidity, senselessness, insensitivity), or lacking interest or point (vexing, exasperating). 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, where stupidity means one must know they are acting in their own worst interest in that 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 adaptation to new data or existing circumstances."

There are various tests to measure intelligence quotient (IQ), as well as tests that measure aptitude, such as the United States Marine Corps' required General Classification Test (GCT), and the Army General Classification Test.

Researchers Michael Klein and Matthew Cancian have reported a declining aptitude among college educated applicants to the Marine Corps over the past 34 years, although this effect was not observed in the general enlisted population.

Researchers Michael J. McFarland, Matt E. Hauer, and Aaron Reuben report those born between 1951 and 1980 may have lost an average of 2.6 IQ points from exposure to leaded gasoline.