Thus, someone who got the verdict of “Not Proved” went “Scot Free”.

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.

Scot-free, like scot itself, is first found in the thirteenth century. The broader use of the word is attested in the sixteenth century.

To escape pursuers or avoid payment.

Origin

A scot is a Scandinavian word for tax or payment. It came to the UK as a form of redistributive taxation which was levied as early the 13th century. Scot free = tax free

. Many people, especially in the USA, are convinced that the phrase originated with the story of Dred Scott.

The etymology of this phrase shows the danger of trying to prove a case on circumstantial evidence alone. In fact the phrase ‘scot free’ has nothing to do with Dred Scott.

Given the reputation of Scotsmen to be careful with their money we might look to Scotland for the origin of ‘scot free’. Wrong again, but at least we are in the right part of the world now. A scot is a Scandinavian word for tax or payment. It came to the UK as a form of redistributive taxation which was levied as early the 13th century as a form of municipal poor relief. The term is a contraction of ‘scot and lot’. Scot was the tax and lot, or allotment, was the share given to the poor.

Scot as a term for tax has been used since then to mean many different types of tax. Whatever the tax, the phrase ‘scot free’ just refers to not paying one’s taxes.

http://www.phrases.org.uk/meanings/scot%20free.html