Douglas Harper's Etymology Dictionary
1817, French, literally "in the natural state;" originally meaning "uncooked," but used euphemistically for "undressed." See natural (adj.).
Wiktionary
adv. 1 in the nude: in the naked state. 2 naturally. 3 In the plainest or simplest manner. 4 Without makeup and with one's natural hair color.
WordNet
Usage examples of "au naturel".
Finally, in order to conserve his fast-dwindling energies, he shed the superficial guise of humanity and slithered au naturel to the scene of the disaster, pushing aside with his tentacles the sharp rocks and obstructing plant life.
They're also showing them a Tri-D of my brother Jack, au naturel, as a more positive inducement.
No one minds, as though I always show up for after-school activities nearly au naturel.
The nose was au naturel too, now that Nefret had removed the squashed lump of putty.
On the tennis court a giggling quartet was enjoying a sloppy game of doubles au naturel.