fundament

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The buttocks.
• noun The anus.
• noun The natural features of a land surface unaltered by humans.
• noun A foundation, as of a building.
• noun An underlying theoretical basis or principle.

from The Century Dictionary.

• noun Foundation; foundment.
• noun The anus; the vent; the perineal region.

from the GNU version of the Collaborative International Dictionary of English.

• noun obsolete Foundation.
• noun The part of the body on which one sits; the buttocks; specifically (Anat.), the anus.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Foundation.
• noun The bottom; the buttocks or anus.
• noun The underlying basis or principle for a theoretical or mathematical system.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun lowest support of a structure
• noun the fleshy part of the human body that you sit on
• noun the fundamental assumptions from which something is begun or developed or calculated or explained