This paper argues that studying why and when people call certain actions stupid should be the interest of psychological investigations not just because it is a frequent everyday behavior, but also because it is a robust behavioral reflection of the rationalistic expectations to which people adjust their own behavior and expect others to. The relationship of intelligence and intelligent behavior has been the topic of recent debates, yet understanding why we call certain actions stupid irrespective of their cognitive abilities requires the understanding of what people mean when they call an action stupid. To study these questions empirically, we analyzed real-life examples where people called an action stupid. A collection of such stories was categorized by raters along a list of psychological concepts to explore what the causes are that people attribute to the stupid actions observed. We found that people use the label stupid for three separate types of situation: (1) violations of maintaining a balance between confidence and abilities; (2) failures of attention; and (3) lack of control. The level of observed stupidity was always amplified by higher responsibility being attributed to the actor and by the severity of the consequences of the action. These results bring us closer to understanding people's conception of unintelligent behavior while emphasizing the broader psychological perspectives of studying the attribute of stupid in everyday life.