1. the buttocks or rear end of a person
Usage: informal
2. the back or rear part of something

Examples

• The children slid down the hill on their backsides.
• She fell and landed hard on her backside.
• The backsides of the houses faced the alley.
• He examined the backside of the painting for the artist’s signature.
• The backside of the mountain was covered in snow.
• They painted the backsides of the fence posts white.