It is used primarily to call attention to the fact that computers will unquestioningly process the most nonsensical of input data and produce nonsensical output, and is a pun on FIFO (First In, First Out).
It was most popular in the early days of computing, but applies even more today, when powerful computers can spew out mountains of erroneous information in a short time.
The actual term "Garbage in, Garbage out", coined as a teaching mantra by George Fuechsel, an IBM 305 RAMAC technician/instructor in New York, soon contracted to the acronym GIGO.
Early programmers were required to test virtually each program step and cautioned not to expect that the resulting program would "do the right thing" when given imperfect input.
The underlying principle was probably cited by Charles Babbage, inventor of the first programmable device who said:
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