I just looked it up. Congratulations, you've just learned a new word for your rear end!

Here's the definition from Webster's Online Dictionary:
1. The fleshy part of the human body that you sit on.
Source: WordNet 1.7.1 Copyright © 2001 by Princeton University. All rights reserved.
Etymology: Nates \Na"tes\, plural noun. [Latin expression, the buttocks.].