1. (n.) Foundation.

2. (n.) The part of the body on which one sits; the buttocks; specifically (Anat.), the anus.

backside base basement basis beam bearing wall bed bedding bedrock behind belly bottom bottom side breech buttocks derriere downside floor flooring fond footing foundation fundamental ground grounds groundwork hardpan lower side lowest layer lowest level nether side nethermost level pavement posterior principle radical rear riprap rock bottom rudiment rump seat sill solid ground solid rock stereobate stylobate substratum substruction substructure terra firma underbelly underbuilding undercarriage undergirding underlayer underneath underpinning underside understruction understructure