This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
峗 combines 山 (mountain) with 危 (danger), but the exact historical development is uncertain. It may have originally referred to a dangerous mountain or a specific place name.
A mountain 山 combined with danger 危 suggests a precarious peak, but no reliable meaning is known.
For ガイ, imagine a guy climbing a dangerous mountain: guy -> ガイ, and the mountain looks risky.
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