sandal
蹝 centers on a type of footwear: a sandal. The meaning is narrow and concrete, with no clear abstract extensions.
蹝 combines 足 (foot) with 徙, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound for a type of footwear.
The foot radical 足 on the left points to footwear. Picture the right side 徙 as a person moving lightly in sandals: the whole character 蹝 is a sandal.
For シ, imagine a simple sandal made of a single sheet of material: sheet -> シ, and the sandal is just a sheet under your foot.