Disappointing, this one - I was hoping it had some raiding and pillaging in England and getting away origin:
"11. SCOTLAND AND SCOT-FREE
Scot-free is an alteration of shot-free, where shot was a charge or share of a payment. It was a lucky thing to get out of a meal or a night at the tavern shot-free. It later came to mean escaping without injury, and came to be pronounced as scot instead of shot."