belly vs buttock

belly

noun
  • The part of anything which resembles (either closely or abstractly) the human belly in protuberance or in concavity; often, the fundus (innermost part). 

  • The stomach. 

  • The main curved portion of a knife blade. 

  • The abdomen, especially a fat one. 

  • The womb. 

  • The hollow part of a curved or bent timber, the convex part of which is the back. 

  • The lower fuselage of an airplane. 

verb
  • To swell and become protuberant; to bulge or billow. 

  • To cause to swell out; to fill. 

  • To position one’s belly; to move on one’s belly. 

buttock

noun
  • Each of the two large fleshy halves of the posterior part of the body between the base of the back, the perineum and the top of the legs. 

  • The convexity of a ship behind, under the stern. 

How often have the words belly and buttock occurred in a corpus of books? (source: Google Ngram Viewer )