What is the X in X-mas?
To process each element of a list of things. "Hold on, I've got to loop through my paper mail." Derives from the computer-language notion of an iterative loop; compare "cdr down" (under cdr), which is less common among C and Unix programmers. ITS hackers used to say "IRP over" after an obscure pseudo-op in the MIDAS PDP-10 assembler.