lice, vermin
蝨 centers on small parasitic insects that infest and cause discomfort: lice and similar vermin.
蝨 combines 卂 (possibly a phonetic or semantic element) with 䖵 (two insects), suggesting a creature associated with insects. The exact historical development is uncertain, but the character likely represents a type of insect or vermin.
The top part 卂 looks like a quick, darting shape, and the bottom 䖵 shows two insects. Picture lice darting quickly among the insects, infesting and causing discomfort.
For シツ, imagine a louse sitting on a sheet: the word 'sheet' sounds like シツ, and the louse is the vermin on it.