Here's why Back to the Future's Doc Brown shouts "Great Scott!" during the movie trilogy. When it comes to classic movies from the 1980s, few are as beloved as Back to the Future, the movie that put star Michael J. Fox on the A-list and solidified the career of director Robert Zemeckis. Back to the Future also ensured Christopher Lloyd would have a career in Hollywood as long as he wanted one, due to Lloyd's Doctor Emmett Brown instantly becoming a movie icon.
While lots of jokes have been made in recent years about how odd it is that Doc Brown's best friend is a non-family-related teenage boy, there's no doubt that Doc and Marty McFly's friendship is completely genuine. Marty, likely at least partially due to how much of a loser his father George ended up in the movie's original timeline, sees Doc as a sort of mentor, an older father figure he can turn to for help and advice, as well as someone with a unique perspective on life.
Doc Brown is truly unlike any other movie character before him. One of Doc's most unique traits is his tendency to exclaim "Great Scott!" when surprised by something for either or good bad reasons - essentially his version of "Eureka!" While some might be inclined to believe the saying was invented for Back to the Future, that's actually not the case, as its use goes much farther back. While it's unclear exactly why Back to the Future director Zemeckis and writer Bob Gale - who sadly have no plans for Back to the Future 4 - went with "Great Scott!" for Doc Brown's trademark phrase, it was born out of a belief that the character should have one.
Notably, there's a Superman tie here, as they almost had Doc say "Great Caesar's Ghost!" instead, which Perry White would often exclaim on the 1950s Superman TV show. For what it's worth, Superman also said "Great Scott!" quite a few times in DC comics of the 1940s and '50s. As for the phrase itself, it's generally agreed that "Great Scott!" was originally a reference to Winfield Scott, a top general in the U.S. Army in the 1800s, and also later a candidate for U.S. President from the Whig Party. He didn't win though. Scott was revered for his military exploits, and also quite large and imposing, making him literally a "great Scott" in that sense.
Over time, "Great Scott!" became a minced oath, which is basically an expletive that's been softened to be less objectionable, such as "gosh darn" instead of "god damn" or "heck" for "hell." Another common exclamation of the time was "Great God!," which some found offensive, leading to the switch of God to Scott. While Back to the Future's Doc Brown obviously wasn't around in the 1800s - until he traveled back there in his DeLorean time machine anyway - it makes sense that a man so unique would adopt a saying that had long since dropped out of common use by his own adulthood. Either that, or he read Superman comics in the '40s.