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

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]