fundament

(noun) the fundamental assumptions from which something is begun or developed or calculated or explained. Synonyms: basis, base, foundation, groundwork, cornerstone. Examples: "The whole argument rested on a basis of conjecture."

(noun) the fleshy part of the human body that you sit on. Synonyms: buttocks, nates, arse, butt, backside, bum, buns, can, hindquarters, hind end, keister, posterior, prat, rear, rear end, rump, stern, seat, tail, tail end, tooshie, tush, bottom, behind, derriere, fanny, ass. Examples: "He deserves a good kick in the butt." "Are you going to sit on your fanny and do nothing?."

(noun) lowest support of a structure. Synonyms: foundation, base, foot, groundwork, substructure, understructure. Examples: "It was built on a base of solid rock." "He stood at the foot of the tower."