Origin and history of Scot

Old English Scottas (plural) "inhabitants of Ireland, Irishmen," from Late Latin Scotti (c. 400), a name of uncertain origin, perhaps from Celtic (but answering to no known tribal name; Irish Scots appears to be a Latin borrowing). The name followed the Irish tribe which invaded Scotland 6c. C.E. after the Romans withdrew from Britain, and after the time of Alfred the Great the Old English word described only the Irish who had settled in the northwest of Britain.

"royal tax," a term that survived in old law and in scot-free; late Old English, "municipal charges and taxes," also "a royal tax or contribution sometimes levied for support of local officers." This is from Old Norse skot "contribution," etymologically "a shooting, shot; a thing shot, a missile" (from PIE root *skeud- "to shoot, chase, throw"). The Old Norse verb form, skjota, has a secondary sense of "transfer to another; pay." It is related to Old English sceotan "to pay, contribute," Middle English scotten "to bear one's share of;" Dutch schot, German Schoß "tax, contribution."

Also via Old French escot "reckoning, payment" (Modern French écot "share"), and via Medieval Latin scotum, scottum, both from Germanic, as is Spanish ecote. 

From c. 1300 as "payment for food or drink at a social gathering," also figurative (late 12c.), a sense also in the Old French word. Hence scot-ale (n.) "a drinking party, probably compulsory, held by a sheriff, forester, bailiff, etc., for which a contribution was exacted" [Middle English Compendium], attested from late 12c., with ending as in bridal. "Scot implies a contribution toward some object to which others contributed equally" [Century Dictionary].