Where did Scott Free originate?
Scot free has no connection with Scotsmen, Scotland, Scottish or me. It’s an accidental connection, just as it is in hopscotch.
Scot is from an Old Norse word that meant a payment (tax) or contribution and which is linked to the modern French écot, a share of communal expenses, as in payer son écot, to pay one’s share. It is a close relative of shot, which at one time could have the same meaning of a contribution or a share of expenses. Some account its original spelling as skot.
The expression scot free derives from a medieval municipal tax levied in proportional shares on inhabitants, often for poor relief. This tax was called a scot, as an abbreviation of the full term scot and lot, where scot was the sum to be paid and lot was one’s allotted share. (This tax lasted a long time, in some places such as Westminster down to the electoral reforms of 1832, with only those paying scot and lot being allowed to vote.) So somebody who avoided paying his share of the town’s expenses for some reason got off scot free.
Scot was also used for a payment or reckoning, especially one’s share of the cost of an entertainment; when one settled up, one “paid for one’s scot”. Again, someone who evaded paying their share of the tab got off scot free.
It’s been suggested that this usage may have come from the old habit of noting purchases of drinks and the like by making marks on a slate, or scotching it, but the evidence suggests this is just a popular etymology.