The Jargon File (version 4.4.7, 29 Dec 2003):

case mod


    [from ?case modification?]

    1. Originally a kind of hardware hack on a PC intended to support 
    overclocking (e.g. with cutouts for oversized fans, or a freon-based or
    water-cooling system).

    2. Nowadays, similar drastic surgery that's done just to make a machine
    look nifty. The commonest case mods combine acrylic case windows with LEDs
    to give the machine an eerie interior glow like a B-movie flying saucer.
    More advanced forms of case modding involve building machines into weird
    and unlikely shapes. The effect can be quite artistic, but one of the
    unwritten rules is that the machine must continue to function as a
    computer.