backside vs fanny

backside

• 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. 

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

• Approaching an obstacle backward 

fanny

• The buttocks; arguably the most nearly polite of several euphemisms. 

• The female genitalia. 

• Women viewed as sexual objects. 

• Mess kettle or cooking pot. 

• Sexual intercourse with a woman.