This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
暟 combines 日 (sun) with 豈, but the exact historical development and original meaning are uncertain. The character is rare and its etymology is not well established.
With 日 (sun) on the left and 豈 on the right, imagine a bright, showy sun that seems to be showing off its light. This ties to the kun-reading てらい (showiness), but the overall meaning remains unclear.
For カイ, picture a bright sun (日) shining over a vast, open sea (海, かい), but the sun is being especially showy (てらい) about it: 海 -> カイ.