Green’s Dictionary of Slang

diddly-squat n.1

also diddley do, diddly, diddly doo, piddly-squat, squat diddley do
[diddly n.1 (2)]

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

[UK]R. Frede Entry E (1961) 85: Kramer shot craps for a while and I was winning piddly-squat.
[US]H. Selby Jr Requiem for a Dream (1987) 16: Just cause they cut the bone outta yours dont mean diddly to me.
[US]G.V. Higgins Patriot Game (1985) 89: We do diddly-squat on one judgeship that was just a formality.
[US]S. Morgan Homeboy 21: They aint got squat diddley do, girl.
[US]L. Bing Do or Die (1992) 125: All your dead homeboys? Even they don’t mean diddly. Because nothing changes.
[US]N. McCall Makes Me Wanna Holler (1995) 230: Bryan didn’t know diddly-squat about the streets.
[UK]N. Barlay Curvy Lovebox 79: He ain’t gonna know diddly doo.
OnLine Dict. of Playground Sl. [Internet] diddly squat n. (1) ignorance i.e. ‘He doesn’t know diddly squat about (whatever)’.
[UK]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’.
[US]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.