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.
Americanized spelling of German Büffel: 1. from late Middle High German büffel ‘buffalo’ (in some areas ‘bull’); the surname may have arisen from a house name or a nickname for a loutish person. 2. from a pet form of the Old German personal name Bodefrit.