It's a kind of magic word in the Marvel universe. It's the word Billy Batson utters to transform himself into Captain Marvel. It's an acronym; it gives him the wisdom of Solomon, the strength of Hercules, the stamina of Atlas, the power of Zeus, the courage of Achilles, and the speed of Mercury, S-H-A-Z-A-M. It's spoken as a word, with the accent on the second syllable: sha-ZAM! Solomon is a character from the Bible, specifically the Old Testament (sacred to both Christians and Jews). King Solomon was a king famous for wisdom. All the others are figures from Greek mythology. I half-knew this, but couldn't remember what the letters in SHAZAM stood for; I found the answers and a lot more in the Wikipedia article, https://en.wikipedia.org/wiki/Captain_Marvel_(DC_Comics)

I think it comes from two words--Shucks and Damn. I heard it in TV shows in the 60's like the Andy Griffith show such. It's definitely slang and probably a combo of two words. Now that I googled it I'm not entirely sure. LOL

It is exclamation used to introduce an extraordinary deed, story or transformation

Jerry, thanks for your comment! As you say, few people are likely to know the word nowadays, so I must refrain from using it.

It's an old nonsensical, funny word. You wouldn't use it in real life. It dates from approximately the 1960s.