Definition of backsides:
(n) :
(euphemistic) A person's buttocks.
(n) :
The back side of anything, the part opposite its front, particularly:
(n) :
The back side of an estate: the backyard and outbuildings behind a main house, especially (UK dialect, euphemistic) an outhouse.
(n) :
(figuratively) The reverse or opposite of anything.
(adj) :
(board sports) Approaching an obstacle backward.
(n) :
Used other than figuratively or idiomatically: see back, side.