oes·trus I. \ˈestrəs, ˈēs-\noun Etymology: Latin, gadfly, from Greek oistros gadfly, desire, frenzy — more at ire 1.-esarchaic: a biting or tormenting fly 2.capitalized[New Latin, from Latin]: the type genus of Oestridae comprising the sheep botfly II. oroestrum variant of estrus