Okay, cool. Good find.
From the May 2000 issue of Communications Technology:
"The subject of the origin of the RG-x designation for coaxial cables came up in an SCTE-List discussion in June 1998. The consensus seemed to indicate that "RG" stands for radio grade or radio guide, but the question wasn't really resolved. Rex Porter, editor-in-chief of IC's sister publication Communications Technology, commented that RG originally stood for radio (government) grade. As the commercial sector began to manufacture RG-type cables, the "government" designation was dropped in favor of radio grade. This was in part because of the concern that RG-designated cables that had not been tested for compliance with MIL-C-17 might be interpreted as MIL-SPEC cables when in fact they were not, so they were simply called RG-type, with RG standing for radio grade.
When this discussion appeared on the SCTE-List, I did a little research and came up with the following. I tracked down the "Army-Navy List of Preferred Cables," which was derived from "RF Transmission Lines and Fittings," MIL-HDBK-216 (4 January 1962, revised 18 May 1965). As a side note, requirements for listed cables can be found separately in MIL-C-17.
The "Army-Navy List of Preferred Cables" didn't specifically define the acronym "RG" but included 75-ohm cables (the common impedance for coaxial cables used in cable TV networks) among the many listed. These were classified as "JAN Type RG-" cables. RG was shown as an indicator, applicable to the family name "cables, RF bulk."
Even after digging up this information, there was still some question as to just what "RG" means. I then contacted Edwards Publishing, the company that published "The Encyclopedia of Connectors," a multi-volume reference series based on industry standards, MIL-SPEC documents and similar material. “RG" was an arbitrary designation for the family name "cables, RF bulk," much like "UG" was an arbitrary designation for the family name "connectors," according to an individual with whom I spoke at Edwards
A contributor to the SCTE-List discussion on this subject referenced a late 1960s version of the "RCA Field Engineers Technical Manual" and found this description:
Component indicator: RG
Family name: cables and transmission lines, bulk, RF
Definition: RF cable, waveguides, etc. without terminals
Yet another SCTE-List subscriber said a US Navy spokesperson told him that the designations were indeed arbitrary, because the military supposedly had huge lists of multi-letter designations for all kinds of materials, supplies, hardware and parts. That cables wound up with the RG designation, was a coincidence, according to that particular source.
Whether or not the designations really are arbitrary is open to debate. Anecdotal evidence suggests RG originally was an acronym for radio government (and later radio grade); the UG designation for connectors is "universal grade;" and the /U in many of the RG cable part numbers simply stands for "universal." Even so, I'm going to have to say the jury is still out on this one.