fundament

noun

1. 1

   lowest support of a structure

   it was built on a base of solid rock

   he stood at the foot of the tower

2. 2

   the fleshy part of the human body that you sit on

   he deserves a good kick in the butt

   are you going to sit on your fanny and do nothing?

3. 3

   the fundamental assumptions from which something is begun or developed or calculated or explained

   the whole argument rested on a basis of conjecture