How do you say grandmaster in Japanese?

there is no Japanese Word for grandmaster cuz im a Grandmaster in taekwon-do i also check no Word

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Taekwondo is Korean, not Japanese, so how would that qualify you...?

Anyway, 達人 (tatsujin) means 'master' or 'expert' in Japanese.

In terms of a martial art, 師範 (Shihan) would mean 'master' and is an honorific that is a high enough level that it can be used interchangeably with 'grandmaster'.