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}]