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 (the same IRP op can nowadays be found in Microsoft's assembler).