From this article comes a nice, succinct definition of “stupidity”:
The Third Basic Law assumes, although it does not state it explicitly, that human beings fall into four basic categories: the helpless, the intelligent, the bandit and the stupid. It will be easily recognized by the perspicacious reader that these four categories correspond to the four areas I, H, S, B, of the basic graph (see below).
If Tom takes an action and suffers a loss while producing a gain to Dick, Tom’s mark will fall in field H: Tom acted helplessly. If Tom takes an action by which he makes a gain while yielding a gain also to Dick, Tom’s mark will fall in area I: Tom acted intelligently. If Tom takes an action by which he makes a gain causing Dick a loss, Tom’s mark will fall in area B: Tom acted as a bandit. Stupidity is related to area S and to all positions on axis Y below point O.
A stupid person is a person who causes losses to another person or to a group of persons while himself deriving no gain and even possibly incurring losses.
When confronted for the first time with the Third Basic Law, rational people instinctively react with feelings of skepticism and incredulity. The fact is that reasonable people have difficulty in conceiving and understanding unreasonable behaviour. But let us abandon the lofty plane of theory and let us look pragmatically at our daily life. We all recollect occasions in which a fellow took an action which resulted in his gain and our loss: we had to deal with a bandit. We also recollect cases in which a fellow took an action which resulted in his loss and our gain: we had to deal with a helpless person. We can recollect cases in which a fellow took an action by which both parties gained: he was intelligent. Such cases do indeed occur. But upon thoughtful reflection you must admit that these are not the events which punctuate most frequently our daily life. Our daily life is mostly, made of cases in which we lose money and/or time and/or energy and/or appetite, cheerfulness and good health because of the improbable action of some preposterous creature who has nothing to gain and indeed gains nothing from causing us embarrassment, difficulties or harm. Nobody knows, understands or can possibly explain why that preposterous creature does what he does. In fact there is no explanation – or better there is only one explanation: the person in question is stupid.
Note that I’m in no way implying that I’m NOT stupid 🙂