Source : Free On-Line Dictionary of Computing

stiffy
     
         ({University of Lowell}, Massachusetts) A
        3.5-inch {microfloppy}, so called because their jackets are
        more rigid than those of the 5.25-inch and the (obsolete)
        8-inch {floppy disk}.  Elsewhere this might be called a
        "firmy".
     
        [{Jargon File}]
     
        (1994-11-03)