lice, vermin
虱 centers on small parasitic insects that infest bodies or living spaces: lice and similar vermin.
虱 is a variant form of 蝨, combining the insect radical 虫 with a phonetic element. The modern form 虱 simplifies the inner component to 䖝, but the exact historical development is uncertain.
The outer shape ⺄ suggests a crawling creature, and the inner 䖝 looks like a bug. Together they form a louse: a tiny vermin that crawls on you.
For シツ, imagine a louse sitting on your skin: sit -> シツ, and the louse is the vermin you want to get rid of.
louse (esp. a sucking louse); lice
very thorough search (for contraband, escaped convict, etc.); fine-tooth-comb search; exhaustive search; scouring
Pediculus humanus (species of louse that infests humans); body louse; body lice; head louse; head lice
bed bug (Cimex lectularius); bedbug
psylla (any insect of family Psyllidae); psyllid; jumping plant louse
mulberry psyllid (Anomoneura mori)
body louse (Pediculus humanus humanus); body lice
whitefly (Aleyrodidae spp.)
biting louse (any louse of order Mallophaga); chewing louse
erect hedge parsley (Torilis japonica); Japanese hedge parsley; upright hedge parsley
lousy; lice-ridden; covered in lice
one by one; thoroughly; exhaustively; with a fine-tooth comb
milkfish (Chanos chanos)
louse fly (any fly of family Hippoboscidae)
body louse (Pediculus humanus humanus); body lice
head louse (Pediculus humanus capitis); head lice
Japanese fish louse (Argulus japonicus)