“To go scot free” means to be exempt from payment, punishment, or penalty. Except in this expression, and in the somewhat rare “to pay one’s scot,” this meaning of “scot” is almost unknown in America.
But it comes straight from Old English; it then meant, as now, a payment, or, especially, one’s share in the cost of some entertainment; later it came to mean also a tax.
Hence, “to go scot free” is, literally, to be free of payment or tax.
