This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
胲 combines ⺼ (flesh/body part) with 亥, which likely contributes a phonetic element. The exact historical development is uncertain.
The left side ⺼ suggests a body part, and the right side 亥 looks like a twisted shape. Imagine a thumb (おやゆび) being the body part that twists and bends the most.
For カイ, picture a kayak paddle held by the thumb: kayak -> カイ, and the thumb grips the paddle.