Per Webster's: A "cracker jack" is a person who does something especially well. It is descended from "crackajack," a rhymed compound from "crack" when used as an adjective for excellent.
Per
Symbols of America, by Hal Morgan: Cracker Jack, the popcorn and peanut candy, was named from the slang term crackerjack, for excellent, which had entered the language in 1896. The candy was first made by FW Rueckheim, a German immigrant in Chicago, who opened a popcorn stand in 1872. The business first sold Crackerjack at the Columbian Exposition in 1893. It was marketed in the Sears catalogue in 1902, and became part of song lyrics in
Take Me Out to the Ballgame in 1908. The little boy in the sailor suit on the package was modeled on Rueckheim's grandson, Robert, and the dog after the boy's dog, Bingo. Robert died of pneumonia soon after the new box with him on it appeared in 1919.
Per Charles Funk, in
Horse Feathers & Other Curious Words: When used to describe certain residents of the southern US, "Crackers" was derived from a use going back to before 1509, when it described a person given to boasting, tall stories or lying. The term was being applied to ignorant and shiftless people in the southern US by 1766, and evolved into a term which southerners apply to themselves now without necessarily pejorative implications.
"Crackpot" comes from the use of "pot" to refer to the head.
