Green’s Dictionary of Slang

(US) a fool.

1848		Bartlett Dict. Americanisms 233: nimshi. A foolish fellow, or one who habitually acts in a foolish manner. Local in Connecticut.
1853		Turnover 60: Why, any nimshi can jump acrost that little crick.
1872		Schele De Vere Americanisms 619: Nimshi, is the Connecticut term for nincompoop.
1909		J. Ware Passing Eng. of the Victorian Era 182/1: Nimshes (American Federal, 1860-65). One of the contemptuous names describing the Secessionists. Origin not known.
1915		M.G. Hayden ‘Terms Of Disparagement’ in DN IV:iii 203: nimshi, foolish fellow. ‘Why any nimshi can jump across that little creek.’.
1944		N.Y. Herald Trib. 24 Sept. 2/4: There were ‘Nimshis’—which word describes the village nitwits, who were treated with tolerance and kindness, even when they barged into the front row of group photographs [DA].
1977		R. Coover Public Burning (1979) 651: ‘Jehu Nimshi!’ he bellowed.