The part of one's back on which one rests in sitting
She slapped his backside playfully.
He slapped her backside again.
He slapped her on the backside playfully as she passed.
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.
He put the girl down and spatted her backside playfully.