Birthday suit

George embarrassed his aunt by answering the door in his birthday suit.

Source: wiktionary

Nature, the universal tailor, provides for every other animal a birthday suit of clothes which is to outlast his life; but man, though so peculiarly sensitive to the assaults of the elements, is ushered into this nether sphere without any covering, in order that he may be compelled to exert his faculties, and become his own tailor, and thereby his own civilizer.

Source: wiktionary

At last I'm down to the nitty-gritty, my original birthday suit. Where do I put my hands? I need pockets.

Source: wiktionary

The terms naturalistic and naturalism suffer from the fact that they have a range of meanings, covering a variety of things from the Romantic submission to nature or the collecting of butterflies to the practice of bathing on the beach clad only in one's birthday suit.

Source: wiktionary