species of water plant
莙 refers to a specific kind of water plant. Its meaning is narrow and botanical, without broader abstract extensions.
莙 combines the grass radical 艹 with 君, which likely contributes a phonetic element. The exact historical development is uncertain.
The top 艹 is grass, and the bottom 君 looks like a ruler. Imagine a ruler standing among water grasses, identifying this particular species.
For キン, picture a kingfisher (キン) diving among the water plants: king -> キン, and the bird hunts near the 莙.