Many people often ask where this phrase originated and whom it refers to. Unfortunately, the origin of this phrase is uncertain.
War Slang, by Paul Dickson, Pocketbooks © 1994, described "Great Scott" as an allusion to General Winfield Scott (1786-1866), American hero of the War of 1812 and the Mexican War in 1847. He was the Whig candidate for President in 1852 and campaigned with great swagger and vanity. He was jeered as "Great Scott" during the campaign, which he lost to Franklin Pierce. Both this book and the Oxford English Dictionary also cite the term as "an expression of surprise", but the Oxford English Dictionary goes no further.
A Browser's Dictionary, by John Ciardi, published as A Common Reader Edition by The Akadine Press, 1980, doesn't approve of the allusion of the expression "Great Scott" to Winfield Scott. A Common Reader Edition indicates that "Great Scott" is derived from the German expression "Gruess Gott!" and that it has been an Americanism only since the 19th Century. This suggests a borrowing from the greetings exchanged by German Immigrants, their cordiality contributing to the exclamatiory sense of the American adaptation.