I first heard the phrase in the Eddie Murphy movie “48 Hours” (1982). A friend tells me it goes back much farther than that, and that a U.S. president might have something to do with it (he couldn’t recall who or how). Google and the usual word-lover websites have failed me.
What say you, O teeming Doper masses?
I remember my father using it when I was in high school, so very early '70s.
The first cites I can find in a quick newspaper database search show it from 1930. More than that, you’re on your own.
Jed Clampet (Buddy Ebson) of the “Beverly Hillbillies” TV sitcom made the phrase popular in the mid-sixties. The shows wrighters must have gotten it from an earlier source like Sam Clem mentioned. WAG, sounds like something Will Rodger’s would have said and that would fit Jed’s character.