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

Intel 486SX
486SX

    An Intel 486DX microprocessor with its
   floating-point unit disconnected.  All 486SX chips were
   fabricated with FPUs.  If testing showed that the CPU was OK
   but the FPU was defective, the FPU's power and bus connections
   were destroyed with a laser and the chip was sold cheaper as
   an SX, if the FPU worked it was sold as a DX.

   [Was this true of all 486SX chips?]

   Some systems, e.g. Aopen 486SX, allowed a DX to be plugged
   into an expansion socket.  A board jumper would disable the SX
   which was hard to remove because it was surface mounted.

   Some SX chips only had a 16-bit wide external data bus.  The
   DX has a pin to select the data bus width (16 or 32).  On the
   smaller SX, that line is hard-wired to 16 inside the
   package.  This is similar to the 286 SX, which was a 16-bit
   processor with an 8-bit external data bus.

   The Jargon File claimed that the SX was deliberately
   disabled crippleware.  The German computer magazine, "c't",
   made this same theory the basis of an April Fools Joke.
   They claimed that if one drilled a hole of a specified
   diameter through the right point on a SX chip, this would
   brake the circuit that disables the FPU.  Some people actually
   tried (and then bought themselves new processors).

   (1997-02-14)