‘gadfly, Tabanus bovinus’ (X 300,0-., Arist.), also of a water-insect and a bird… ‘sting’ (S., E.), ‘rage, madness, fierce desire’ (Hdt., PI., S., E.)
Beekes establishes that oistros encompasses the literal gadfly, its physical sting, and — crucially — rage, madness, and fierce desire, grounding the psychological valence of the term in its very etymology.
, Etymological Dictionary of Greek, 2010thesis