This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 昦 is uncertain. It visually combines 日 (sun) with 夰, but the original meaning and component function are unclear.
With 日 (sun) above and 夰 (a large, spread-out shape), imagine a vast, bright sky, but the true meaning is uncertain.
For コウ, picture a vast, empty sky (そら) stretching out like a huge dome: the open space echoes the sound コウ, like the English word 'co-' in 'cosmos'.