This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 夰 is uncertain. It visually contains 大 on top, but the origin and original meaning are unclear.
With no clear meaning, picture 大 (big) on top of a simple lower shape, like a person standing tall but whose purpose is unknown.
For コウ, imagine a big (大) person named Kou standing on a platform: Kou -> コウ, but what they are doing is a mystery.