What is the meaning of fundament?

noun

• The fundamental assumptions from which something is begun or developed or calculated or explained.
  • usage: "the whole argument rested on a basis of conjecture"
  • synonyms: basis, base, foundation, groundwork, cornerstone
• The fleshy part of the human body that you sit on.
  • usage: "he deserves a good kick in the butt"; "are you going to sit on your fanny and do nothing"
  • 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
• Lowest support of a structure.
  • usage: "it was built on a base of solid rock"; "he stood at the foot of the tower"
  • synonyms: foundation, base, foot, groundwork, substructure, understructure