GIGO (garbage in, garbage out) is a concept common to computer science and mathematics: the quality of output is determined by the quality of the input. So, for example, if a mathematical equation is improperly stated, the answer is unlikely to be correct. Similarly, if incorrect data is input to a program, the output is unlikely to be informative.
George Fuechsel, an early IBM programmer and instructor, is generally given credit for coining the term. Fuechsel is said to have used "garbage in, garbage out" as a concise way of reminding his students that a computer just processes what it is given. The term is now widely used in computer science classes, IT services and elsewhere. In fact, GIGO is sometimes used to refer to situations in the analog world, such as a faulty decision made as a result of incomplete information.
A variation on the term, "garbage in, gospel out," refers to a tendency to put unwarranted faith in the accuracy of computer-generated data.