Green’s Dictionary of Slang

(orig. US) nothing, zero; thus it don’t mean diddly-squat, it is totally irrelevant, unimportant.

1958		R. Frede Entry E (1961) 85: Kramer shot craps for a while and I was winning piddly-squat.
1978		H. Selby Jr Requiem for a Dream (1987) 16: Just cause they cut the bone outta yours dont mean diddly to me.
1982		G.V. Higgins Patriot Game (1985) 89: We do diddly-squat on one judgeship that was just a formality.
1990		S. Morgan Homeboy 21: They aint got squat diddley do, girl.
1991		L. Bing Do or Die (1992) 125: All your dead homeboys? Even they don’t mean diddly. Because nothing changes.
1994		N. McCall Makes Me Wanna Holler (1995) 230: Bryan didn’t know diddly-squat about the streets.
1997		N. Barlay Curvy Lovebox 79: He ain’t gonna know diddly doo.
2001		OnLine Dict. of Playground Sl. 🌐 diddly squat n. (1) ignorance i.e. ‘He doesn’t know diddly squat about (whatever)’.
2009		T. Black Gutted 102: ‘What have you got for me this morning, Mac?’ [...] ‘Some damp in the cludgie I’d say needs looking at smartish. Apart from that, diddly’.
2011		E. Beetner ‘Zed’s Dead, Baby’ in Pulp Ink [ebook] [The] kid on Christmas morning who finds nothing but a pine tree with diddly-squat underneath it.
2021		P Howard Braywatch 53: She knows diddly focking squat about the game.