fundament

1. lowest support of a structure

    e.g. it was built on a base of solid rock
           he stood at the foot of the tower

    Synonym: foundation base foot groundwork substructure understructure

2. the fleshy part of the human body that you sit on

    e.g. he deserves a good kick in the butt
           are you going to sit on your fanny and do nothing?

    Synonym: 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

3. the fundamental assumptions from which something is begun or developed or calculated or explained

    e.g. the whole argument rested on a basis of conjecture

    Synonym: basis base foundation groundwork cornerstone