gad ‧ fly /ˈɡædflaɪ/ BrE AmE noun ( plural gadflies ) [countable]
[ Date: 1500-1600 ; Origin: gad 'pointed stick' (13-19 centuries) (from Old Norse gaddr ) + ↑ fly 2 ]
1 . someone who annoys other people by criticizing them
2 . a fly that bites cattle and horses