“Magnum opus” in Japanese
Short answer
Magnum opus in Japanese is “最高作” (saikousaku). Full meaning: magnum opus, (one's) best work.
Other ways to say “Magnum opus” in Japanese
代表作
daihyousaku most important work (of a writer, artist, etc.), representative work, masterpiece, magnum opus, major work
大著
taicho voluminous work, bulky volume, tome; great work, masterpiece, magnum opus
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