used in transliterations
This rare kanji has no clear standalone meaning. It is used in transliterations, so its meaning is essentially phonetic.
嗶 combines 口 (mouth) with 畢, which likely provides the sound. It was created for phonetic use in transliterations.
The mouth radical 口 suggests a sound, and the right side 畢 gives the pronunciation. Think of 嗶 as a character used to represent a specific sound in foreign words.
For ヒツ, imagine a hissing sound 'hitsu' coming from the mouth 口 when pronouncing a transliterated word: hitsu -> ヒツ.
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