Well, Stales, I didn't know this one. In checking through numerous references, including a listing of the 1828 MW, it turns out that originally these naturists found all meaning in nature--nature was spirit and provider of all. Then it segued into meaning a person who received health benefits from nature. And now, in the latest edition of AHD, at least, there is only one single meaning and that is the single, bare, wind-whipped, sun-glowing figure of the nudist. The old 1828 meaning has disappeared altogether...

Bare regards,
Wordwrapped

PS: Is there any expression "in his altogether" meaning he's nude? Or have I gotten something mixed up?