backside vs sit upon

backside

noun

  • A person's buttocks. 

  • Used other than figuratively or idiomatically: see back, side. 

  • The back side of an estate: the backyard and outbuildings behind a main house, especially (UK dialect, euphemistic) an outhouse. 

  • The reverse or opposite of anything. 

adj

  • Used other than figuratively or idiomatically: see back, side. 

  • Approaching an obstacle backward 

sit upon

noun

  • buttocks, bottom 

  • small, portable cushion for sitting on 

verb

  • Synonym of sit on (“block, suppress, hold back”) 

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