This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 畡 is uncertain. It visually combines 田 (field) and 亥, but the original semantic and phonetic roles are unclear.
Since the meaning is unclear, picture a field 田 beside a mysterious 亥 shape, like an unknown boundary marker at the edge of a field.
For カイ, imagine a field 田 where someone shouts 'kai!' to mark the edge: kai -> カイ, and the sound echoes across the uncertain boundary.