Why do the Scots start chanting "Maccaulich" after their first victory?

In the script of the movie Braveheart (Section 9) the Scots are chanting "MacAulish, MacAulish, WALLACE, WALLACE!" The term "Mac" means "son of", and "Aulish" is intended to be a variation of the Medieval Gaelic name "Uallas" (later translated to the Anglican, "Wallace"). Where the writers got the spelling "Aulish" is anyone's guess. Perhaps they were just trying to simplify the name for literary purposes. At any rate, essentially the Scots are chanting "MacUallas!" or "Son of Wallace" which then morphs into "Wallace!"

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