falling on one's backside; pratfall
mochi used to celebrate a child's first birthday; mochi tied to a baby's back if he starts walking before his first birthday in order to cause him to fall on his backside
氷で滑って尻餅をついた。
I slipped on the ice and fell on my backside.
芸人が派手な尻餅で観客を笑わせる。
The comedian made the audience laugh with an exaggerated pratfall.
よちよち歩きの赤ちゃんが、何度も尻餅をついている。
The wobbly toddler keeps falling on their bottom.