fundament

the fundamental assumptions from which something is begun or developed or calculated or explained; "the whole argument rested on a basis of conjecture"

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?"

lowest support of a structure; "it was built on a base of solid rock"; "he stood at the foot of the tower"