If you slip on ice and fall on your rear end, then that is an example of falling on your backside.
The backside of the building faced an alley, and was covered in grime and scrawled graffiti.
After riding the horse all day for the first time, I had painful blisters on my backside.
She slapped his backside playfully.
Any other day, she'd have stared at his hard body and the way his jeans hugged his muscular thighs and the round globes of his backside, or the T-shirt that fit so well.
Her clothes were soaking wet and her backside was numb with cold.
He rolled up the newspaper and hit her playfully on the backside as she walked away.