This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
吤 combines 口 (mouth) with 介, which likely contributes a phonetic element. The exact historical development is uncertain.
With 口 (mouth) beside 介, imagine a mouth making a sound like 'kai', but the meaning is unclear.
For カイ, imagine a mouth (口) calling out 'kai!' like a seagull's cry: kai -> カイ.