Green’s Dictionary of Slang

diddly-squat n.1

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

[1950s+] (orig. US) nothing, zero; thus it don’t mean diddly-squat, it is totally irrelevant, unimportant.