This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 琹 is uncertain. It visually contains 玨 (a pair of jade pieces) and 木 (tree/wood), but the original semantic or phonetic motivation is unclear.
The top 玨 looks like two jade pieces, and the bottom 木 is a tree. Without a clear meaning, picture a mysterious object made of jade and wood.
For キン, imagine a king (キン) inspecting the mysterious jade-and-wood object: king -> キン.