bit cup
鋂 is a rare kanji whose given meaning is a bit cup, a metal part of a horse's bridle. Its standalone meaning is narrow and technical.
鋂 combines 金 (metal) with 每, which likely contributes the sound. The exact historical development is uncertain.
The left side 金 means metal, and the right side 每 looks like a mother or every. Picture a metal cup that every horse needs as part of its bridle: a bit cup.
For バイ, imagine buying a metal bit cup: buy -> バイ, and the metal part is what you purchase.