This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
碊 combines 石 (stone) with 戔, which likely contributes a phonetic element. The exact historical development is uncertain.
With 石 (stone) on the left and 戔 on the right, picture a stone tablet with an unknown inscription. The meaning is uncertain, like a forgotten word carved in rock.
For セン, imagine a stone (石) with a sen (千) tiny scratches: 千 -> セン, and the stone holds a thousand uncertain marks.