louse eggs, nits
蟣 centers on the tiny eggs of lice, commonly called nits. It refers specifically to these parasitic insect eggs.
蟣 combines 虫 (insect) with 幾, which likely contributes a phonetic element. The character is used for louse eggs, fitting the insect radical.
The insect radical 虫 on the left hints at a tiny creature, while the right side 幾 suggests something small and numerous. Picture a cluster of minuscule louse eggs clinging to a hair.
For キ, imagine a key that unlocks a magnified view of tiny nits: key -> キ, and through the lens you see the louse eggs clearly.