bad at horse-riding; unsteady rider
Describes someone who cannot sit steadily on a horse and bounces or fidgets in the saddle. The image is of a peach-shaped bottom that won't stay still.
He's a peach-bottom—he's bad at horse-riding.
fidgety person; restless person
Extended from the horse-riding sense: someone who can't sit still, constantly shifting or squirming. Often used humorously.
That kid is a fidget-bottom—he can't sit still even during class.
From 桃 (peach) + 尻 (buttocks). The image of a peach-shaped bottom that won't stay still gave rise to the meanings 'bad at horse-riding' and, by extension, 'fidgety person'.