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.
English: nickname for someone with gray hair or a gray beard, from Old English græg ‘gray’. In Scotland and Ireland it has been used as a translation of various Gaelic surnames derived from riabhach ‘brindled’, ‘gray’ (see Reavey). In North America this name has assimilated names with similar meaning from other European languages.