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. [Jargon File]