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

The Free On-line Dictionary of Computing (30 December 2018):

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]