http://dictionary.cambridge.org/dictionary/english/tertius

a Latin word meaning "third", used in medical names and descriptions

http://www.merriam-webster.com/medical/peroneus%20tertius

a branch of the extensor digitorum longus muscle that arises especially from the lower portion of the fibula, inserts on the dorsal surface of the base of the fifth metatarsal bone, and flexes the foot dorsally and assists in everting it