This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
硞 combines 石 (stone) with 告, which likely contributes a phonetic element. The exact historical development is uncertain.
The stone radical 石 suggests something related to rock or hardness, but the overall meaning is obscure. Picture a stone tablet with the character 告 carved into it, its message lost to time.
For カク, imagine a stone (石) that makes a sharp 'kaku' sound when struck: kaku -> カク.
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