Douglas Harper's Etymology Dictionary
cracker-jack
also crackerjack, "something excellent," 1893, U.S. colloquialism, apparently a fanciful construction, earliest use in reference to racing horses. The caramel-coated popcorn-and-peanuts confection was said to have been introduced at the World's Columbian Exposition (1893). Supposedly a salesman gave it the name when he tasted some and said, "That's a cracker-jack," using the then-popular expression. The name was trademarked 1896. The "Prize in Every Box" was introduced 1912.\n\n"Your brother Bob is traveling, isn't he?"\n
\n"Yep. He's with one of the big racing teams. I tell you, he's a cracker-jack! Wins a bushel of diamonds and gold cups every week."\n
["Life," Aug. 1, 1895]
Wiktionary
cracker-jack
a. (alternative spelling of crackerjack English)