Throughout English, tiger was taking its place as it competed with other forms like tyger, tigre, and tygre. All four of these variations come from the Old English word tigras, from French tigre, which in turn derives from Latin tigris, still meaning "tiger". This is likely the source of the name of the famous Mesopotamian river, as well. Tigris is from its Greek cognate tigris, which most likely goes back to the Avestan word for "arrow", tigri (since a tiger pounces on you like an "arrow", all quick and deadly). Avestan, being an Iranian language (in which Zoroastrian scripture was written), has a lot of its words go back to Old Persian, and this is no exception. Tigri is theorized to trace from tigra, a word that meant "sharp" or "pointed" and has an obvious connection to "arrow". Rather than going back to Indo-Iranian and Proto-Indo-European, this then takes a turn into another language family, towards Sumerian, but that's all we know.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.