Source : Free On-Line Dictionary of Computing

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)