From Longman Dictionary of Contemporary Englishgadflygad‧fly /ˈɡædflaɪ/ noun (plural gadflies) [countable] 1 CRITICIZEsomeone who annoys other people by criticizing them2 TAHBIa fly that bites cattle and horses
Examples from the Corpus
gadfly• A gadfly, a maverick, a treasured pain in the posterior.• These people are inconsequential, but they are gadflies.• Hirschfield, a wealthy political gadfly, spoke at the conference.• Which had not included Rory, the gadfly son.Origin gadfly (1500-1600) gad “pointed stick” ((13-19 centuries)) (from Old Norse gaddr) + FLY2 