Answers:
from randomhouse.com
"Scot-free actually comes from scot, meaning 'an assessment or tax'. This word is a borrowing from Old Norse, perhaps from scot 'contribution' or a related word scattr 'tax; treasure'. It also seems to be related to Old English gescot 'shot', and there is also the possibility of the influence of Old French escot, which was itself borrowed from a Germanic source. Scot is first found in this sense in the early thirteenth century.
Scot-free originally meant 'free from payment of scot', a sense that is now almost totally obsolete, except in historical contexts. By extension, it came to mean 'free from obligation, harm, punishment, or restraint'. (Yes, the idea that a tax is a punishment is a very old one.) Since the word scot on its own is now rare, most people probably interpret scot to be some sort of intensive of free."