A gadfly is a term that may encompass several fly species that bite or irritate either livestock or people. Figuratively, a gadfly is someone who irritates or annoys, specifically by constant criticism. Sometimes this criticism is for the purpose of moving someone or a group of people into action.
The plural is gadflies.
The term comes from the word gad which has several meanings. It can be a name or an interjection. As a verb, it means to move to and from different locales purely to find entertainment. It is informal in this definition.
Some confusion may arise from readers who know the word gad but are less familiar with gadfly. They may erroneously assume a gadfly flits from place to place seeking pleasure.
Also, we couldn’t find a good example of the term used in the literal sense. This may be lack of use in formal writing, or decline in popularity.
The average gadfly lasts just a few days. Nigel Farage has thrived for 11 years since Michael Howard gave him that label, also dismissing him as a “crank”. [The Telegraph]
Nobody tried harder than Ernie Kovacs, an early television gadfly who pushed boundaries as a comedian, actor, writer and producer. [Houston Chronicle]
These gadflies like to show up and throw hand grenades about the place, trying to force the board of directors to do things that most of them don’t want to do: buy back shares; merge with other companies; ditch certain board members; the list goes on. [Marketplace]