Green’s Dictionary of Slang

(US) a nosey, inquisitive person.

c.1960		in IUFA Folk Speech n.p.: Denotes someone who is overly curious, adj. nibby noun: nib or nib-shit [HDAS].
1979		G. Wolff Duke of Deception (1990) 235: The banker was a ‘nibshit’.
2000		Justme.org 28 Aug. [blog] He’ll want to go so he can be a ‘nibshit’ as my Gram used to call me. (A nibshit is a kid who can’t stand to know things are going on that he can’t see and they get their noses into everything just in case so they don’t miss something.).
2004		J. Headley Small Town Odds 264: ‘Hey, mind your own business, nibshit,’ barked Mac.