[This is a fast-access FAQ excerpt.]
Like "hopscotch", this word for "without incurring any penalty"
has no connection with frugal Scotsmen. In 12th-century England, a
"scot" or "sceot" was a municipal tax paid to the local bailiff or
sheriff (the word came from an Old Norse cognate of "shoot"/"shot",
and meant "money thrown down"). The word "scot-free", which is
recorded from the 13th century, referred to someone who succeeded in
dodging these taxes. Later, the term was given wider currency when
"scot" was used to mean the amount owed by a customer in a tavern:
anyone who had a drink on the house went "scot-free". This "scot"
was reinforced by the fact that the drinks ordered were "scotched",
or marked on a slate, so that the landlord could keep track of how
much the customer owed.