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formal; stiff; strict; ceremonious
彼は堅苦しい話し方で説明した。
He explained it in a stiff, formal manner.
堅苦しく考えなくていい。
You don't need to think about it so rigidly.
会の雰囲気は予想以上に堅苦しかった。
The atmosphere at the gathering was more formal than expected.