Английский Этимологический словарь - gadfly (n.)

- 1626, "fly which bites cattle," probably from gad "goad, metal rod" (1250), here in the sense of "stinger," from O.N. gaddr "spike, nail," from P.Gmc. *gadaz "pointed stick;" but sense is entangled with gad (v.) and an early meaning of gadfly was also "someone who likes to go about, often stopping here and there."