to display the teeth
齞 centers on the act of showing or baring the teeth, as in a grin or snarl.
齞 combines 齒 (teeth) with 只, which likely contributes a phonetic element. The character expresses the idea of displaying the teeth.
The left side 齒 shows a mouth full of teeth, and the right side 只 looks like a mouth open wide. Together they suggest baring the teeth for all to see.
For ゲン, imagine a wide grin that makes you think of a genial (friendly) person: genial -> ゲン, and the grin shows the teeth.
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