This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
垬 combines 土 (earth) with 共, but the exact historical development and original meaning are uncertain.
The left side is earth 土 and the right side is 共, but without a clear meaning, picture a mound of earth shared among people.
For コウ, imagine a shared mound of earth in a public park: 公 (こう, public) -> コウ, tying the earth radical to a common reading.