caulk, spider
蟢 centers on a spider, specifically one that spins webs or seals crevices. The 'caulk' meaning is a rare, specialized extension of this sealing behavior.
蟢 combines 虫 (insect/creature) with 喜, which likely contributes the sound. The character is used for a type of spider, but the exact historical development is uncertain.
A joyful (喜) insect (虫) that spins webs to seal gaps: a spider that caulks its home.
For キ, imagine a spider sealing a crack with a key (キー) made of silk: key -> キ, and the spider caulks the crevice.
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