The Jargon File (version 4.4.7, 29 Dec 2003):

loop through
 vt.

    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).


The Free On-line Dictionary of Computing (30 December 2018):

loop
loop through

    A sequence of instructions in a program that
   the processor repeats, either until some condition is met, or
   indefinitely (an infinite loop).

   In an structured language (e.g. C, Pascal, BASIC, or
   Fortran), a loop is usually achieved with for loop, while
   loop or repeat loop constructs.

   In other languages these constructs may be synthesised with a
   jump (assembly language) or a GOTO (early Fortran or
   BASIC).

   To "loop through" a list means to process each element in turn.
   Hackers might use this for everyday iterative actions, e.g.  "I've
   got to loop through my paper mail."

   (2014-12-20)