Sources : Wether

Isidore of Seville [7th century CE] (Etymologies, Book 12, 1:10): The wether [vervex] is either named from force [vis, viris], because it is stronger than the other sheep, or because it is male [vir], that is, masculine; or because it has a worm [vermis] in its head - irritated by the itching of these worms they butt against each other and strike with great force when they fight. - [Barney, Lewis, et. al. translation]