Green’s Dictionary of Slang

(US) an excellent person or thing.

1885		Frankfort Roundabout (KY) 16 May 2/2: Two lightning jerkers [...] are working at the Western Union telegraph ofice [...] Col. Terhune, the gentlemanly manager, says they are ‘Jim-dandies’.
1887		Courier-Journal (Louisville, KY) 12 Jan. 2/1: Dear Sir: Though a stranger to you (yet a Democrat), let me say you are a ‘Jim Dandy’ [DA].
1895		E. Townsend Chimmie Fadden Explains 8: It was bout Miss Fannie’s weddin I was goin t’tell you. Say, it was a Chim dandy.
1905		G.W. Peck Peck’s Bad Boy Abroad 55: You made the longest shot ever made on the Potomac. It was a Jim dandy, you old frog eater.
1905		L. Pound ‘Dialect Speech in Nebraska’ in DN III:i 62: jim dandy, joe dandy, n. Intensive of dandy, as term of approval or admiration. ‘Your knife is a jim dandy’.
1919		Van Loan ‘Piute vs. Piute’ in Score by Innings (2004) 318: It was a Jim-dandy of a peg, neither too high nor too low, but just exactly right.
1922		‘He Loves Sugar and Tea’ in T.W. Talley Negro Folk Rhymes 84: Mistah Buster, he’s a Jim-dandy! / He can swing dem gals so handy.
1933		N. West ‘Miss Lonelyhearts’ in Coll. Works (1975) 270: This one is a jim-dandy.
1949		H. Miller Sexus (1969) 483: He is as fresh as a daisy, dapper, cool, insolent – a real Jim Dandy.
1953		A.J. Liebling Honest Rainmaker (1991) 100: He was a jim-dandy and always well occupied.
1961		G.L. Coon Meanwhile, Back at the Front (1962) 151: It must have been a jim-dandy.
1977		R. Coover Public Burning (1979) 517: And that word ‘integer’ was a jimdandy, too!
1983		J. Ciardi A Second Browser’s Dict. 157: Jim-dandy. An outstanding person or thing. The Kid was a jim-dandy.