noun, plural gad·flies.
- any of various flies, as a stable fly or warble fly, that bite or annoy domestic animals.
- a person who persistently annoys or provokes others with criticism, schemes, ideas, demands, requests, etc.
noun plural -flies
- any of various large dipterous flies, esp the horsefly, that annoy livestock by sucking their blood
- a constantly irritating or harassing person
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.) 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).