As used by hackers, specifically describes a person who habitually reasons from obviously or demonstrably incorrect premises and cannot be persuaded by evidence to do otherwise; it is not generally used in its other senses, i.e., to describe a person with a native incapacity to reason correctly, or a clown. Indeed, in hackish experience many fools are capable of reasoning all too effectively in executing their errors. See also cretin, loser, fool file.
The Algol 68-R compiler used to initialize its storage to the character string "F00LF00LF00LF00L..." because as a pointer or as a floating point number it caused a crash, and as an integer or a character string it was very recognizable in a dump. Sadly, one day a very senior professor at Nottingham University wrote a program that called him a fool. He proceeded to demonstrate the correctness of this assertion by lobbying the university (not quite successfully) to forbid the use of Algol on its computers. See also DEADBEEF.