Green’s Dictionary of Slang

(US) anything or anyone insignificant, unimportant.

1959		‘Andrew Shaw’ Campus Tramp 76: ‘And the page two make-up [...] it was so lousy. You must have taken all the little diddley-shit that was left over and stuck it on page two ’.
1964		Poston ‘Problems in the Study of Campus Sl.’ in AS XXXIX:2 117: The discreet abbreviation of possibly offensive words or phrases; bull- and diddly-shit become bull and diddly.
1973		G.V. Higgins Digger’s Game (1981) 108: He’s still sweating the diddly-shit he gets from the other.
1977		J. Sayles Union Dues (1978) 188: I sell ass. I don’t do diddley shit unless there’s some coin connected to it.
1989		H. Rawson Dict. of Invective (1991) 352: diddly shit. A measure of insignificance, applied to anything of little or no value; a.k.a. diddly poo, diddly squat, or just plain diddly.
2004		(con. 1960s-70s) T. Taylor Top Fellas 53/1: Bowie’s proclamations of sexual versatility meant diddly-shit to most sharps.
2012		J. Shaw ‘Luck’ in ThugLit Sept./Oct. [ebook] ‘That wouldn’t prove diddly-shit, dumbass’.