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Engetsu (剡月) means scathing moon. 剡 seems to be an exceedingly rare character in Japanese. Kubo seems fond of giving words and characters readings that knowingly resemble--and so evoke--real words, but have different meanings, e.g. Zangetsu "slaying moon" resembles zangetsu "waning/morning moon" (more lit. "lingering moon"), and Engetsu "scathing moon" resembles the word engetsu "crescent moon" (more lit. "dammed [and so curtailed] moon") cited above. (The above is quoted from the Bleach Wiki's Translation Corner.)