Douglas Harper's Etymology Dictionary
jim-dandy
"remarkable person or thing," 1844, perhaps from an old song, "Dandy Jim of Caroline" (1840s).
Wiktionary
jim-dandy
a. (context chiefly US colloquial English) excellent, outstanding. alt. (context chiefly US colloquial English) excellent, outstanding. n. (context chiefly US colloquial English) Something that is a very superior example of its kind of thing.