This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
啿 combines 口 (mouth) with 甚, which likely contributes the sound. The exact historical development is uncertain, and the character is rare.
With 口 (mouth) beside 甚, imagine a mouth speaking with great intensity or excess, but the exact meaning is unclear.
For タン, picture a tan-colored mouth speaking intensely: tan -> タン.