The Jargon File (version 4.4.7, 29 Dec 2003):

EXCH
 /eks'ch@/, /eksch/, vt.

    To exchange two things, each for the other; to swap places. If you point to
    two people sitting down and say ?Exch!?, you are asking them to trade
    places. EXCH, meaning EXCHange, was originally the name of a PDP-10
    instruction that exchanged the contents of a register and a memory
    location. Many newer hackers are probably thinking instead of the 
    PostScript exchange operator (which is usually written in lowercase).


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

EXCH

    /eks'ch*/ or /eksch/ To exchange two things, each for
   the other; to swap places.  If you point to two people sitting
   down and say "Exch!", you are asking them to trade places.
   EXCH, meaning EXCHange, was originally the name of a PDP-10
   instruction that exchanged the contents of a register and a
   memory location.

   Many newer hackers are probably thinking instead of the
   PostScript exchange operator (which is usually written in
   lowercase).

   [Jargon File]

   (1999-09-17)