This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
鈅 combines the metal radical 金 with the phonetic component 月. The exact historical development and original meaning are uncertain.
The metal radical 金 suggests something metallic, and 月 (moon) might hint at a crescent shape, but without a clear meaning, it's hard to form a reliable mnemonic.
For ゲツ, imagine a metal crescent moon (月) that you get (ゲツ) as a gift. The moon shape and the sound 'get' cue ゲツ.
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