This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
滈 combines 氵 (water) with 高, but the exact historical development is uncertain. It may have originally referred to a body of water or a place name, but the modern form is rare and its origin is not well documented.
The left side is water 氵, and the right side is tall 高. Imagine a tall waterfall or a deep, high-water mark, but the meaning is uncertain.
For コウ, imagine a tall 高 (こう) wave of water 氵: 高 -> コウ, and the water rises high.