"Shazam!" was originally what Billy Batson said to become Captain Marvel
With its February 1940 issue of Whiz Comics, Fawcett Publications introduced Billy Batson, a homeless orphan who follows a stranger into an abandoned subway tunnel (those were some seriously innocent times) and meets the ancient wizard Shazam. Shazam gives Billy the ability to summon the powers of six conveniently named gods and heroes -- Solomon, Hercules, Atlas, Zeus, Achilles, and Mercury -- by shouting their names. The nonsense word summons a lightning bolt that turns him into a red-and-yellow-costumed grownup named Captain Marvel, and saying "Shazam!" turns him back into normal Billy Batson.
Captain Marvel quickly became a phenomenon, and the character's success rivaled even that of Superman, who had debuted a year-and-a-half earlier. In addition to a live-action 1941 movie serial (Superman wouldn't get the serial treatment until 1948), Captain Marvel appeared in every issue of Whiz and spawned several spinoffs, among them Captain Marvel Adventures, which published two best-selling issues every month. These additional titles showcased not only the Captain, but also "Marvel family" members Mary Marvel (Billy's sister) and Captain Marvel Jr., whose alter-ego was Freddy Freeman, a newsboy with whom Captain Marvel shared his powers after the subtly named villain Captain Nazi had maimed the kid.
Freddy's most-enduring qualities were a cape-and-slight-pompadour look that inspired a young Elvis Presley, and the awkward aspect of his magic word: While Billy and Mary said "Shazam!" to transform, Freddy had to say the source of his powers -- that is, Captain Marvel -- to change into Captain Marvel Jr. That meant Captain Marvel Jr. was the first superhero who couldn't say his own name (otherwise he'd constantly change forms), an unwieldy bit of business that would eventually define the entire franchise.