The suffix attached to crippled models of older x86 processors; 386SX, 486SX. These were standard models of the processors, minus the floating point co-processor. The more conspiracy-oriented computer users are fairly convinced that it was a marketing scam by Intel to make people upgrade from SX to DX chips, and those more trusting of Intel believe that it was just a way of opening the bargain market. The processor cost in fact about the same to make, and sold for less, so that Intel actually had a smaller profit margin on them. While at the time it was generally regarded as a screwup, it opened the bargain market they could later introduce the Celeron in.
The other day on eBay (which I browse when bored), I spotted a 486SX desktop offering "affordable speed." Bahahahahahah.