also gad-fly, 1620s, "fly which bites cattle," probably from gad (n.) "goad, metal rod," here in the sense of "stinger;" but the sense is entangled with gad (v.) "rove about" (on the notion, perhaps, of the insect's power of flight or of the restlessness of animals plagued by them), and another early meaning of gadfly was "someone who likes to go about, often stopping here and there" (1610s). Sense of "one who irritates another" is from 1640s (equivalent of Latin oestrus; see estrus). "In strictness, only the females are gadflies, the males being smaller and quite inoffensive, living on juices of plants" [Century Dictionary]. Earlier bot-fly, from bot "skin parasite" (late 15c.).
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