In the 1500s, the term scot meant a fine or tax. To “get off scot-free” originally meant to “be free from paying a fine or tax.” Nowadays, scot-free means being “completely free from obligation, harm, or penalty.”
However, scot is often confused with the term Scotch. Scotch originated as a contraction of Scottish in the 15th century, meaning someone who is a native or inhabitant of Scotland. Unless you’re referring to an absence of Scottish people, remember to always use scot-free.
Example: There wasn’t enough evidence for the jury to convict the defendant, so he got off scot-free.
Sources:
Christine Ammer, The American Heritage Dictionary of Idioms, (Boston: Houghton Mifflin Harcourt, 2013), 166.
Webster's Third New International Dictionary, Unabridged, s.v. “Scotch,” accessed February 8, 2019, http://unabridged.merriam-webster.com.
Webster's Third New International Dictionary, Unabridged, s.v. “scot-free,” accessed February 8, 2019, http://unabridged.merriam-webster.com.