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}]