Probably a corruption of Tur aie! 'Thor, aid,' a battle-cry of the Norsemen. - Wace. *
The word is very generally now (ca. 1888), and was formerly invariably, spelt huzza. The pronunciation in Pope's time, and even until living memory, was hurray. The following couplet from the 'Rape of the Lock,' shows the current pronounciation at the time the poem was written:
One self-approving hour whole years outweighs
Of stupid starers, and of loud hazzas.
Ed note: Important to note the pronounciation discrepancies if one finds themselves inadvertently the passenger in a time machine whereby they end up in a locale predating 1888. Otherwise, you would be seen immediately as an inter-time interloper. The teeth wouldn't give it away at all. No way.
* This has Victorian romanticism written all over it (i.e. "I pulled this out of my corset"). In fact, Catherine Lucy Wilhelmina Powlett Cleveland's The Battle Roll (1889) seems to further this madness:
Thor-aide the old Scandinavian cri de guerre once in general use among the Northmen which was exchanged for Dex aie (Dieu-aide), their war cry at the battle of Hastings. (p163).