fundament

1. the fundamental assumptions from which something is begun or developed or calculated or explainedExamples: "the whole argument rested on a basis of conjecture"Synonyms: base, basis, cornerstone, foundation, fundament, groundwork

2. the fleshy part of the human body that you sit onExamples: "he deserves a good kick in the butt"; "are you going to sit on your fanny and do nothing?"Synonyms: arse, ass, backside, behind, bottom, bum, buns, butt

3. lowest support of a structureExamples: "it was built on a base of solid rock"; "he stood at the foot of the tower"Synonyms: base, foot, foundation, fundament, groundwork, substructure, understructure