fundament

nouns

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