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讨人厌者；有意搅扰他人者;（尤指）故意挑战权威者
If you refer to someone as a gadfly, you believe that they deliberately annoy or challenge other people, especially people in authority.
例句
It still remains to be seen whether Buchanan will be just a gadfly or a real threat to the president.
布坎南只是故意搅局还是对总统构成真正的威胁，这还有待观察。