N-COUNT 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.
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。