Green’s Dictionary of Slang

1. (Irish) a small monetary payment; thus the diddley club, a savings club, used by the poor, to which one could contribute as little as one halfpenny a week.

1981		(con. c.1920) P. Crosbie Your Dinner’s Poured Out! 145: Don’t forget the Diddley, / Last week we didn’t pay.
1994		(con. 1930s) K.C. Kearns Dublin Tenement Life 32: The most popular means by which tenement women saved money for Christmas was the ‘diddley club’. [...] ‘My mother was always in the diddley. My aunt ran one. It was for saving the money, before credit unions.’.
1995		P. O’Keeffe Down Cobbled Streets, A Liberties Childhood 139: The Aunt Bridie was up Saturday. She just got her summer diddly and she was handin’ out money like snuff at a wake .

2. nothing whatsoever.

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.
1987		‘Joe Bob Briggs’ Joe Bob Goes to the Drive-In 8: A bunch of farmers and barbers and feed-store cowboys that didn’t know diddly about the movie binness.
1994		J. Wambaugh Finnegan’s Week 156: He didn’t seem to know diddly.
1994		N. McCall Makes Me Wanna Holler (1995) 325: Young white boys who didn’t know diddly.
1996		J. Byrell Lairs, Urgers & Coat-Tuggers 189: [T]hat didn’t mean diddly to this bloke.
2000		F. Kellerman Stalker (2001) 412: She doesn’t know diddly about hiring a professional popper.
2007		N. McCall Them (2008) 11: She didn’t give a squirrel’s butt about people; she didn’t know diddly, and couldn’t care less.
2016		A. Wheatle Crongton Knights 18: Jonah didn’t say diddly about it so we rolled on.
2020		A. Trebek The Answer Is 267: When I started doing crossword puzzles, I couldn’t do them for diddly.

3. (orig. US black) anything unimportant or insignificant; usu. in phr. not give a diddley, couldn’t care less.

1979		implied in diddley-damn
1987		‘Joe Bob Briggs’ Joe Bob Goes to the Drive-In 197: What the hey, the man’s in jail, he don’t give a diddly.
1996		J. Cameron It Was An Accident 85: They only passed on a message, didn’t know diddly themselves.