1626, "fly which bites cattle," probably from
gad "goad, metal rod" (c.1225), 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 1649 (equivalent of L.
oestrus).