This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
銈 combines 金 (metal) with 圭, which likely contributes the sound. The exact historical development is uncertain.
The left side is metal 金, and the right side is 圭 (a jade tablet). Picture a metal tablet whose purpose is unclear, reflecting the kanji's uncertain meaning.
For ケイ, imagine a metal 金 key 圭: key -> ケイ, but the key's purpose is a mystery.