The seat; the posteriors; the rear end.

And the LORD said, Like as my servant Isaiah hath walked naked and barefoot three years for a sign and wonder upon Egypt and upon Ethiopia; so shall the king of Assyria lead away the Egyptians prisoners, and the Ethiopians captives, young and old, naked and barefoot, even with their BUTTOCKS uncovered, to the shame of Egypt. ( Isaiah 20:3-4 )