Source : WordNet®

sidecar
     n 1: a cocktail made of orange liqueur with lemon juice and
          brandy
     2: conveyance consisting of a small carrier attached to the
        side of a motorcycle

Source : Free On-Line Dictionary of Computing

sidecar
     
        1. Synonym {slap on the side}.  Especially used of add-ons for
        the late and unlamented {IBM PCjr}.
     
        2. The {IBM PC} compatibility box that could be bolted onto
        the side of an {Amiga}.  Designed and produced by {Commodore},
        it broke all of the company's own design rules.  If it worked
        with any other peripherals, it was by {magic}.
     
        [{Jargon File}]