1620s, "fly which bites cattle," probably from gad "goad, metal rod" (early 13c.), 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." Sense of "one who irritates another" is from 1640s (equivalent of L. oestrus).