This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
鋋 combines 金 (metal) with 延, which likely contributes the sound. The exact historical development is uncertain.
The metal radical 金 suggests something made of metal, but the overall meaning is obscure. Picture an ancient, forgotten metal tool whose purpose is lost to time.
For セン, imagine a thousand (千, せん) ancient metal coins scattered beside the forgotten tool: 千 -> セン, and the metal 金 ties it together.