backside vs rear end

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 

rear end

noun

  • The buttocks. 

  • The back or hindmost part of anything, such as a car. 

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