Who came up with this lovely terminology for that area?
I don’t have an answer, except that it’s probably not the same person who came up with “the sun shines out of our behinds”.
This might shed some light on the area. 
I remember it as a line from the first Bad News Bears movie which would be 1976. imdb doesn’t have it as one of their memorable quotes so my memory could be wrong.
But surely it long predates that (I’d guess 50 years or more).
I think this was the first “catch phrase” slogan for the first personal hygiene item to be advertised around the time of the Model “T”, I WAG! 
The linguists over a ADS-L seem to agree, as of last year. But they don’t search for phrases all that often. I’ve found one that looks promising from a Google Book Search(which is not always valid–no preview). I’ll try to go to the local Univ. library and see a hard copy. It’s from 1943. But, of course, it could have been used many years before that. It’s not the kind of thing that makes it into print in general.
I know I heard the phrase in the mid-60s. BNB may have been the first time in the general media, but it predates that by some time.
Another vote for “Bad News Bears”. It may not be the first time it was used, but that’s where I first remember hearing it.
I suspect the basic idea was an established phrase when it made this TV appearance in 1971, in this notorious confrontation between Norman Mailer and Dick Cavett (page down to “Celebrity Gone Wild”) – but Cavett may have done a bit of improv with the “fold it five ways” and “moon” (instead of sun) details.