BACKSIDE

[noun] [C] [informal] - the part of the body that you sit on; your bottomAfter cycling for the whole day, my backside was very sore.She's so lazy - she needs a boot/kick up the backside (= someone should tell her to do something).Get off your backside (= Do not be so lazy) and do some work!I do all the work, while all you do is sit (around) on your backside (= do nothing) all day.