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. Definition. 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.