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" (unDERcdr), 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]