As a medieval given name this is a Norman form of the French name Thierry, from Germanic Theodoric, from þeud ‘people, race’ + rīc ‘power, ruler’. This was adopted by the Normans and introduced by them to Britain. In modern English use it seems at first to have been a transferred use of the surname derived from the medieval given name, and later to have been taken as a pet form of Terence.
Italian (Sicily): patronymic form of Scudero, a status name equivalent to Squire, from scudero ‘shieldbearer’, Latin scutarius, an agent derivative of scutum ‘shield’. In Byzantium, the Greek word skoutarios, a derivative of this, denoted a member of the imperial guard.