backside vs sacrum

backside

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

  • Approaching an obstacle backward 

noun
  • 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. 

  • A person's buttocks. 

sacrum

noun
  • A large triangular bone at the base of the spine, located between the two ilia (wings of the pelvis) and formed from vertebrae that fuse in adulthood. 

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