fundament

Noun

• lowest support of a structure
  • it was built on a base of solid rock
    
  • he stood at the foot of the tower
    

• 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?
    

• the fundamental assumptions from which something is begun or developed or calculated or explained
  • the whole argument rested on a basis of conjecture
    

Related words

• Synonyms: 
  foundation , base , foot , groundwork , substructure , understructure , buttocks , basis , cornerstone