sheath
鞞 centers on a protective covering for a blade: a sheath or scabbard, typically made of leather.
鞞 combines 革 (leather) with 卑, which likely contributes a phonetic element. The character originally referred to a leather sheath or scabbard.
The left side 革 means leather, and the right side 卑 looks like a low, flat container. Imagine a flat leather pouch used as a sheath for a blade.
For ヘイ, picture a sheath made of hay: hay -> ヘイ, and the leather sheath is stuffed with hay to protect the blade.