A gadfly is an irritating person, a nuisance who will often ask too many questions or lob lots of criticisms in order to get his or her way.
Gadfly can also refer to an actual fly. The next time you see a cow swatting at the air with her tail, look closer because she might actually be shooing away an annoying gadfly. This is any kind of fly that likes to pester, and even bite, livestock. The prefix gad is an old word for “spike” or “pointed stick” which can be thought of, in this case, as similar to a stinger. So if your office mate is continuously buzzing around your desk with stinging comments, call him a gadfly and try to shoo him back to his own desk.