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successfully; fairly; artfully; nicely; completely; thoroughly
詐欺師にまんまと騙されてしまった。
I was completely fooled by the con artist.
彼はまんまと試験に受かったそうだ。
I heard he successfully passed the exam.
敵の罠にまんまと引っかかる。
To fall neatly into the enemy trap.