Source : WordNet®

GIGO
     n : (computer science) a rule stating that the quality of the
         output is a function of the quality of the input; put
         garbage in and you get garbage out

Source : Free On-Line Dictionary of Computing

GIGO
     
        /gi:'goh/ 1. Garbage In, Garbage Out.  A reference to the fact
        that computers, unlike humans, will unquestioningly process
        the most nonsensical of input data and produce nonsensical
        output.  Of course a properly written program will reject
        input data that is obviously erroneous but such checking is
        not always easy to specify and is tedious to write.
     
        GIGO is usually said in response to {lusers} who complain that
        a program didn't "do the right thing" when given imperfect
        input or otherwise mistreated in some way.  Also commonly used
        to describe failures in human decision making due to faulty,
        incomplete, or imprecise data.
     
        2. Garbage In, Gospel Out.  This more recent expansion is a
        sardonic comment on the tendency human beings have to put
        excessive trust in "computerised" data.
     
        [{Jargon File}]