Brewer's: Great Scott
or Scot! A mitigated form of oath. The initial letter of the German Gott is changed into Sc.
“`Great Scott! ... Beg pardon!' ejaculated Silas, astounded.” —A. C. Gunter: Baron Montez, book iv. chap. xix.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894