straw sandals or slippers that have no heel-backs
屣 centers on a specific type of simple footwear: straw sandals or slippers that lack heel-backs, worn casually or indoors.
屣 combines 尸 (a body or dwelling enclosure) with 徙 (to move, shift), likely suggesting footwear that is easily slipped on and off. The exact historical development is uncertain.
The outer 尸 looks like a foot or a slipper shape, and the inner 徙 suggests movement. Picture a straw sandal that moves easily with your foot because it has no back.
For シ, imagine she slipping on her straw slippers: she -> シ, and the slippers have no heel-backs.