Hephthalite empire, to have one's breath taken
嚈 is a rare kanji with two distinct meanings that do not share a clear conceptual center: it refers to the historical Hephthalite empire, and separately to having one's breath taken away. Treat these as unrelated senses.
嚈 combines 口 (mouth) with 厭, which likely contributes the sound. The exact historical development is uncertain, and the modern form may be a phonetic compound used for a specific name or rare word.
The mouth 口 beside 厭 suggests breath being taken away through the mouth, as if the complex shape of 厭 overwhelms the mouth. For the Hephthalite empire, picture a historical name that uses this rare character.
For ヨウ, imagine a sudden gasp that sounds like 'yo!' when your breath is taken away: yo -> ヨウ, and the mouth 口 is left open in surprise.
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