backside vs tail end

backside

• The reverse or opposite of anything. 

• 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. 

• A person's buttocks. 

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

• Approaching an obstacle backward 

tail end

• The hindmost part of anything (a person, animal, or object), the rear end; the butt, buttocks; hindquarters, rump. 

• The last part of a period of time, event, or situation; the concluding or final part.