slap on the side

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

slap on the side

   (Also called a {sidecar}, or abbreviated "SOTS").  A type of
   external expansion hardware marketed by computer manufacturers
   (e.g. {Commodore} for the {Amiga} 500/1000 series and {IBM} for
   the hideous failure called {IBM PCjr}).  Various SOTS boxes
   provided necessities such as memory, hard drive controllers,
   and conventional expansion slots.

   [{Jargon File}]