From the Wikipedia.
"Gadfly" is a term for people who upset the status quo by posing upsetting or novel questions, or just being an irritant.
The term "gadfly" was used by Plato in the Apology to describe Socrates' relationship of uncomfortable goad to the Athenian political scene, which he compared to a slow and dimwitted horse. The Bible also references the gadfly in terms of political influence; The Book of Jeremiah (46:20, Darby Bible) states "Egypt is a very fair heifer; the gad-fly cometh, it cometh from the north." The term has been used to describe many politicians and social commentators.
During his defense when on trial for his life, Socrates, according to Plato's writings, pointed out that dissent, like the tiny (relative to the size of a horse) gadfly, was easy to swat, but the cost to society of silencing individuals who were irritating could be very high. "If you kill a man like me, you will injure yourselves more than you will injure me," because his role was that of a gadfly, "to sting people and whip them into a fury, all in the service of truth."
In modern and local politics, gadfly is a term used to describe someone who persistently challenges people in positions of power, the status quo or a popular position. For example, Morris Kline wrote "There is a function for the gadfly who poses questions that many specialists would like to overlook. Polemics are healthy." The word may be uttered in a pejorative sense, while at the same time be accepted as a description of honourable work or civic duty.