This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
腇 combines the flesh radical ⺼ with 委, which likely contributes a phonetic element. The exact historical development is uncertain.
The flesh radical ⺼ suggests something related to the body, but the overall meaning is unclear. Imagine a mysterious body part that has been forgotten.
For ダイ, imagine a body part that makes you want to die when it hurts: die -> ダイ, tied to the flesh radical.