“Great Scott!” is an exclamation of surprise or amazement that was popular in the 19th century. The origin is unclear, but “Scott” is used in a similar way to the German “mein Gott” (“My God!” or “Good God!” or “Great God!”) and is a minced oath. “Great Scott!” is dated today, but is still used, often just in jest for people named “Scott.”
 
Scottish historical novelist, poet, playwright and historian Walter Scott (1771-1832) is sometimes thought to have been the person called “Great Scott,” but the exclamation is an Americanism and was not in use during Walter Scott’s lifetime.
 
“Great Scott! is it possible that we ever promised to publish this law” was printed in Spirit of Democracy (Woodsfield, OH) on March 7, 1845. No person named “Scott” was named.
 
“Great Scott!” was frequently associated with American military commander and political candidate Winfield Scott (1786-1856). “The exclamation of ‘great SCOTT’, so frequently used by many people, is said to allude to Gen. Scott, the Whig candidate for President” was printed in the Quincy (IL) Whig on July 19, 1852. It’s often stated that “Great Scott!” was applied to General Winfield Scott during the Mexican-American War (1846-1848), but the 1845 use of “Great Scott!” predates this war.
 
A “Great Scott” possible influence is a poem about Martin Scott that was printed in many newspapers in 1841. The verses include an exclamation (“Great heavens!”) and the term “Great Martin Scott”:
   
“My name is Scott.”
“Great heavens! what?
Is’t Martin Scott?” “The same.
I’ve the tallest luck
In all Kentuck—
I always ‘down’ my game.”
   
“It must be so!
And I’ll come below—
Don’t fire—I will not fly;
I’ve heard of your shot,
Great Martin Scott,
And a gone young ‘coon am I!”
 
The German expression “Grüß Gott” was also an influence on “Great Scott,” although it was a greeting (rather than an exclamation) and means “God bless (you).”
   
     
Wikipedia: Great Scott
“Great Scott!” is an interjection of surprise, amazement, or dismay. It is a distinctive but inoffensive exclamation, popular in the second half of the 19th century and the early 20th century, and now considered dated.
 
It originates as a minced oath, historically associated with two specific “Scotts”, notably Scottish author Sir Walter Scott and somewhat later in the United States, US general Winfield Scott.
     
Wikipedia: Winfield Scott
Winfield Scott (June 13, 1786 – May 29, 1866) was an American military commander and political candidate. He served as a general in the United States Army from 1814 to 1861, taking part in the War of 1812, the Mexican–American War, the early stages of the American Civil War, and various conflicts with Native Americans. Scott was the Whig Party’s presidential nominee in the 1852 presidential election, but was defeated by Democrat Franklin Pierce. He was known as Old Fuss and Feathers for his insistence on proper military etiquette, and as the Grand Old Man of the Army for his many years of service.
 
(Oxford English Dictionary)
Great Scott, int.
Origin: From a proper name, combined with an English element. Etymons: great adj., proper name Scott.
Etymology: <

great adj. + the surname Scott (apparently in the name of General Winfield Scott (1786–1866), United States army general: compare quot. 1852 and see note below), probably as a euphemistic alteration of Great God! at great adj. 17d.
Although this expression has been said to commemorate other individuals with the surname Scott (as e.g. Sir Walter Scott: see Walter Scottish adj.), early evidence appears to support the suggestion given here. Winfield Scott, Commanding General of the United States Army (1841–61) and Whig party presidential candidate (1852), was a popular national figure in the United States in the mid 19th cent., celebrated as a hero for his role in the Mexican-American War (1846–8).
orig. U.S.
Expressing surprise, amazement, annoyance, admiration, etc.
1852   Madison (Indiana) Daily Banner 28 July   The exclamation of ‘great Scott’, so frequently used by many people, is said to allude to Gen. Scott, the Whig candidate for President.
1856   Eclectic Med. Jrnl. Dec. 524/1   ‘Great Scott!’ Mystery upon mystery, and marvel upon marvel!
1871   J. W. De Forest Overland 206   ‘Great—Scott!’ he gasped in his stupefaction, using the name of the then commander-in-chief for an oath, as officers sometimes did in those days.
     
Newspapers.com
14 October 1841, The Daily Picayune (New Orleans, LA), pg. 1, col. 6:
The Gone Coon—AN AMERICANISM.
“‘I’m a gone coon,’ implies ‘I am distressed’ or ‘ruined’ or ‘lost.’ I once asked the origin of this expression, and was gravely told as follows: ‘There was a Captain Martin Scott, in the United States Army, who was a remarkable shot with a rifle. He was raised, I believe, in Vermont. His fame was so considerable through the state, that even the animals were aware of it.’
(...)
One summer’s noon,
A poor raccoon
On a lofty bough had got,
When he saw below
What he guessed was his foe,
For he looked like Captain Scott.
(...)
“My name is Scott.”
“Great heavens! what?
Is’t Martin Scott?” “The same.
I’ve the tallest luck
In all Kentuck—
I always ‘down’ my game.”
 
“It must be so!
And I’ll come below—
Don’t fire—I will not fly;
I’ve heard of your shot,
Great Martin Scott,
And a gone young ‘coon am I!”
C. S.
   
Chronicling America
7 March 1845, The Spirit of Democracy (Woodsfield, OH), “The Bank Bill,” pg. 2, col. 5:
The great fine-and shearing (financiering) scheme of Mr. Kelly, has become the law of the land; and this bill of seventy five sections we promised to publish, should it finally passed into law.  Great Scott! is it possible that we ever promised to publish this law.
   
19 July 1852, Quincy (IL) Whig, pg. 2, col. 5:
The exclamation of ‘great SCOTT’, so frequently used by many people, is said to allude to Gen. Scott, the Whig candidate for President.
             
Google Books
December 1856, The Eclectic Medical Journal, pg. 524, col. 1:
“Oh! Moses! Let no man hereafter presume to say Ex-Prof. Buchanan has not taught, and does not teach, practical Eclecticism. He informs you, Eclectics, that these conditions are of great physiological value, ‘as they are highly applicable to the treatment of disease.’ He tells you the aquatic, or cold blooded condition, is valuable as an antiphlogistic agent, and that it soothes and tranquilizes the lungs. ‘Great Scott!’ Mystery upon mystery, and marvel upon marvel! Will day ever dawn? What does our author mean? The writer is again unexpectedly surrounded by the eternal fogs of our author’s brilliant scientific discoveries, and by the brilliancy of his elocution!”
 
Penn State University Libraries
1 October 1857, Philadelphia (PA) Press, “Speech of Benjamin Rusk, Esq.,” pg. 4, col. 1:
He referred to the famous Dred Scott decision. he once knew an office of the army, who, whenever he became very emphatic, had a habit of invoking his great commander, and was apt to exclaim, “Great Scott!”
     
California Digital Newspaper Collection
10 May 1861, Sacramento (CA) Daily Union, “Letter from New York,” pg. 1, col. 5:
Jolly! Great Scott!
   
Google Books
Eye of the Storm:
A Civil War Odyssey
By Robert Knox Sneden
Edited by Charles F. Bryan, Jr. and Nelson D. Lankford
New York, NY: Touchstone
2002
Pg. 225:
(Diary entry of May 3-8, 1864.—ed.)
“Great Scott,” who would have thought that this would be the destiny of the Union Volunteer in 1861–2 while marching down Broadway to the tune of “John Brown’s Body,” etc?
   
Google Books
Archive.org
Miss Ravenel’s Conversion from Secession to Loyalty
By John William De Forest
New York, NY: Harper & Brothers
1867
Pg. 29:
“I follow General Scott. No Virginian need be ashamed to follow old Fuss and Feathers. We used to swear by him in the army. Great Scott! the fellows said.”
Pr. 40:
“Great Scott! no man can say that.”
   
28 November 1877, Puck (New York, NY), “Puck’s History of the United States,” pg. 7, col. 2:
Santa Anna said: “Up, guards, and at ‘em.” It was not, however, an atom of use, for Santa Anna cried: “Great Scott, he’s got ‘em!”—meaning the heights. “Great Scott” is now used by many people, who, until they read PUCK, will never know what they mean by the exclamation.
 
Google Books
Americanisms—Old & New
Edited by John Stephen Farmer
London, UK: Thomas Poulter & Sons
1889
Pg. 276:
GREAT SCOTT! — A common exclamation of surprise.
     
15 May 1892, Cleveland (OH) Plain Dealer, “A Soldier Hermit. The Secluded Life Now Led By Gen. Alfred Pleasanton. The Latter Days,” pg. 9, col. 5:
Gen. Pleasanton is a man who never profanes his lips with an oath, nevertheless he is not guiltless of the use of epithets. Years ago, before the now familiar exclamation “Great Scott!” dotted the columns of the newspapers of the country, he used it as his most frequent substitute for the expression of a meaning generally conveyed by the use of downright vulgarity and profanity. He was doubtless the inventor of that odd phrase, suggested by the wonderful campaigns of Gen. Scott in Mexico, and it is a pity, perhaps, that he did not file a caveat to cover it, or have it copyrighted for his own especial use and benefit. At any rate it has served the purpose thousands of times, in print and otherwise, of giving vent to both humorous and serious emotions. But his greatest and most forcible epithet is “Great Caesar’s ghost.” When he hurls these words through his teeth, albeit his voice is soft and pleasant to the ears of his listeners, he conveys an impression of earnestness, indignation or surprise that does not reside in any other vehement and italicized expression.
   
Google Books
Dictionary of Phrase and Fable
By Ebenezer Cobham Brewer
London, UK: Cassell and Company, Limited
1895
Pg. 548:
Great Scott or Scot. A mitigated form of Oath. The initial letter of the German Gott is changed into Sc.
   
OCLC WorldCat record
Great Scott! Humorous duet. Written & composed by H. Montague.
Author: Harold Montague
Publisher: London : Reynolds & Co, [1903]
Edition/Format:   Musical score
   
October 1911, Out West, “What and Why is Slang?” by Morris H. Crockett, pg. 240:
The expression “Great Scott” dates back to the Mexican war in which General Winfield Scott distinguished himself and is an example of the tenacity with which a phrase clings to our vocabulary long after the sense has departed from it.
 
OCLC WorldCat record
Great Scott! : a history of northern Wisconsin’s earlier days
Author: Beatrice Durand Derrick
Publisher: Rice Lake, Wis. : Chronotype Publishing, ©1965.
Edition/Format:   Print book : English
   
OCLC WorldCat record
Great Scott! : the best of Jay Scott’s movie reviews
Author: Jay Scott
Publisher: Toronto : McClelland & Stewart, ©1994.
Edition/Format:   Print book : English
 
Urban Dictionary
great scott
A phrase one uses to show surprise or disbelief. Made famous by Doc Brown in the Back to the Future movies.
“Great Scott!” Doc yelled as he dropped a microwave on his toe.
by Honor Man May 03, 2005
 
Slate
OCT. 21 2015 8:02 AM
Great Scott! Who Was Scott? The Origin of Doc Brown’s Favorite Phrase, Explained.
By Forrest Wickman
Today is Back to the Future Day, the day when we finally reach the precise date of Back to the Future II, survey the fact that we got memes instead of hoverboards, and utter Doc Brown’s favorite exclamation: “Great Scott!” But who was Scott?