1
falling on one's backside;pratfall
See also:
尻餅をつく (しりもちをつく)
2
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
Other forms
尻もち 【しりもち】 · 尻餠 【しりもち】 (search-only kanji form)