Source : Free On-Line Dictionary of Computing

big iron
     
         (Or "heavy metal [Cambridge]) Large, expensive,
        ultra-fast computers.  Used generally of {number crunching}
        {supercomputers} such as {Cray}s, but can include more
        conventional big commercial {IBM}ish {mainframes}.  The term
        implies approval, in contrast to "{dinosaur}".
     
        [{Jargon File}]
     
        (2000-11-09)