Crackerjack meaning excellent or superb is an Americanism dating to 1888. Crack-a-jack is a variant. The origin of the term is obscure. From New York Times of 23 April 1888:
The Canadian owner, Mr. Forbes, has Rowland, Brait, and several others in McDonald’s stable, and Charles Miller has four, two of them being 2-year-olds which he expects will become “Cracker Jacks.”
The brand name for the caramel popcorn product comes from the slang usage. The name was trademarked in 1896.
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