sidecar

from WordNet (r) 3.0 (2006)

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

from The Free On-line Dictionary of Computing (8 July 2008)

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}]