The wether is named from the worms in its head
The wether is stronger than other sheep. The wether gets its Latin name vervex from the worms (vermis) in its head, says Isidore of Seville. When wethers are excited by an itch they strike one another with great force.
Isidore of Seville [7th century CE] (Etymologies, Book 12, 1:10): The wether (vervex) is named either from its strength (vires) because it stronger than other sheep; or because it is male (vir) or because it has worms (vermis) in its head. When excited by an itch they strike one another with great force.