chirping, squeaking, hissing
吱 centers on high-pitched, non-verbal sounds made by small creatures or objects: chirping, squeaking, or hissing.
吱 combines 口 (mouth) with 支, which likely contributes the sound. The character is used to represent chirping or squeaking sounds.
A mouth 口 making a high-pitched sound like a branch 支 creaking: chirping, squeaking, or hissing.
For シ, imagine a tiny mouse squeaking 'shee!' as it chirps: shee -> シ.
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