WordNet sense 1 (the fundamental assumptions from which something is begun or developed or calculated or explained):

base, threaten, cornerstone, foundation, groundwork

WordNet sense 2 (the fleshy part of the human body that you sit on):

backside, behind, bottom, bum, buns, buttocks, can, derriere, fanny, hindquarters, posterior, rear, rump, seat, stern, tail, tush, arse, keister, nates, prat, tooshie, tush, tail end, hind end

WordNet sense 3 (lowest support of a structure):

foot, substructure, understructure