fundament in WordNet English dictionary

1. 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?"
   buttocks  nates  arse  butt  backside  bum  buns  can  fundament  hindquarters  hind end  keister  posterior  prat  rear  rear end  rump  stern  seat  tail  tail end  tooshie  tush  bottom  behind  derriere  fanny  ass 
   
2. lowest support of a structure
   "it was built on a base of solid rock"; "he stood at the foot of the tower"
   foundation  base  fundament  foot  groundwork  substructure  understructure 
   
3. the fundamental assumptions from which something is begun or developed or calculated or explained
   "the whole argument rested on a basis of conjecture"
   basis  base  foundation  fundament  groundwork  cornerstone