“Secret method” in Japanese
Short answer
Secret method in Japanese is “秘法” (hihou). Full meaning: secret method, secret process, secret formula; esoteric rituals (in Shingon Buddhism).
Other ways to say “Secret method” in Japanese
秘方
hihou secret method, secret recipe, (prescription for a) secret medicine
秘策
hisaku secret plan, secret scheme, secret stratagem, secret method
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