This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
齖 combines 齒 (tooth) with 牙 (fang/tusk), likely forming a character related to teeth or tusks. The exact historical development is uncertain.
The left side is 齒 (tooth) and the right side is 牙 (fang). Picture a mouth with both regular teeth and sharp fangs, but the exact meaning is unclear.
For ガ, imagine a gap-toothed grin: gap -> ガ, and the toothy shape of 齖 reminds you of the reading.
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