Green’s Dictionary of Slang

1. a fool.

1908

L.W. Payne Jr ‘Word-List From East Alabama’ in DN III:iv 325: jim-crack, n. 1. A sorry sort of a person, a ninny. ‘He’s a regular jim-crack’.

2. a lie.

1908

L.W. Payne Jr ‘Word-List From East Alabama’ in DN III:iv 325: jim-crack, n. [...] 2. A lie. ‘He told me a jim-crack’.