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diligently; assiduously; industriously; beavering away
彼はせっせと働いてお金を貯めている。
He is diligently working and saving money.
蟻がせっせと食べ物を運んでいる。
Ants are busily carrying food.
母はせっせと料理を作って待っていた。
My mother was busily cooking and waiting.