wow, you and I have coincidence of timing. Just as you were posting this:
WAG came is techno-military-speak for wild ass guess.

I was sending this PM: "I've never heard WAG used anywhere but on this board. But I'd occasionally heard the term SWAG, standing for Stupid Wild-Ass Guess. With that in mind it didn't take much to figure out what WAG stood for!"

I hadn't known that this originated in the military. But from what little I know of how soldiers talk (for example, SNAFU), my SWAG is that the original military version was SWAG, with WAG being a cleaned-up derivative version.