Other forms: backsides
The backside is the part of the body you sit on. Sit on your backside, a kick up the backside, land on his backside.
It is mildly informal and not rude, which is why it is the word adults use in front of children. A kick up the backside means the push somebody needs. As two words, the back side of something is simply its rear.
1nounbacksideThe part of the body you sit on.
“He slipped on the ice and landed on his backside.”
backside is a body part.
Also accepted in games: buttocks
Back plus side, recorded in this sense from the sixteenth century.
From back + side.
Etymology from Wiktionary, CC BY-SA