female
牝 centers on the female of a species, especially in contrast to the male. It is used to mark something as female or feminine.
牝 combines 牛 (cow) with 匕, which here likely represents a female marker or a phonetic element. The character was used to denote female animals, especially cattle.
The left side is a cow 牜, and the right side 匕 looks like a ladle or a spoon. Imagine a cow being fed with a special ladle reserved only for the female cows: 牝 means female.
For ヒン, picture a hen clucking around the female cow: hen -> ヒン, and the hen is also female, reinforcing the meaning.