(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.
Try this search on Wikipedia, OneLook, Google
Nearby terms: slammer worm « SLANG « S-Lang « slap on the side » slash » slashdot effect » slave tty