What is the X in X-mas?
/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).