fundament
noun.
the fundamental assumptions from which something is begun or developed or calculated or explained
the whole argument rested on a basis of conjecture
Synonym: base, basis, cornerstone, foundation, groundwork
noun.
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?
Synonym: rear end, rear, prat, posterior, nates, rump, seat, stern, tail, tail end, tooshie, tush, keister, hindquarters, hind end, arse, ass, backside, behind, bottom, bum, buns, fanny, derriere, can, buttocks, butt
noun.
lowest support of a structure
it was built on a base of solid rock|he stood at the foot of the tower
Synonym: base, foot, foundation, groundwork, substructure, understructure