leather arm guard
鞸 refers to a leather arm guard, a protective piece worn on the arm.
鞸 combines 革 (leather) with 畢, which likely contributes the sound. The character is used for a leather arm guard.
革 (leather) on the left suggests something made of leather. The right side 畢 looks like a net or a finished action, but here imagine it as a guard wrapping around the arm: a leather arm guard.
For ヒツ, imagine a leather arm guard that fits snugly: "fits" -> ヒツ. The guard fits perfectly on your arm.