backside
n. countable B1 Intermediate US //ˈbækˌsaɪd// UK //bˈæksaɪd// back·side
n. the part of your body that you sit on. It is a slightly funny or polite way to say 'bottom'.
n. the buttocks or posterior of a person. Often used as a mild euphemism or in a lighthearted, humorous context.
He fell right on his backside while skating.
After sitting in the hard plastic chair for three hours, my backside was completely numb.
The comedian's routine relied heavily on physical humor, including several exaggerated falls that landed him squarely on his backside to the delight of the audience.
From back + side.
Usage
Commonly used in British English as a less vulgar alternative to other terms for the anatomy.