This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 姟 is uncertain. It combines 女 (woman) with 亥, but the original semantic and phonetic roles are unclear.
With no clear meaning, picture a woman 女 beside a mysterious 亥 shape, like a forgotten name from an old story.
For ガイ, imagine a woman named Gai standing beside the 亥 component: Gai -> ガイ.
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