A small or worthless amount.
Origin of diddly-squat
- Alteration of diddlyshit (influenced by doodly-squat) diddly (probably from diddle) shit
From American Heritage Dictionary of the English Language, 5th Edition
- The Random House Historical Dictionary of American Slang suggests that this is a variation of doodly-squat from 1934. Doodly-squat was originally the more common form, but diddly-squat overtook it in the mid-1970s, and is now four times more common in print.