“Diddly-squat” is a small or worthless amount, or nothing.
Example: “She didn’t care diddly-squat about Darryl.”
The word “diddly” itself means “a thing of little or no value, or a flaw, a malfunction.”
The word “squat” is “used as a euphemism for ‘shit,’ which comes from the act of squatting in order to defecate.
In more direct but less polite language, “She didn’t give a shit about Darryl.”
The Random House Historical Dictionary of American Slang suggests that this is a variation of “doodly-squat” from 1934, probably from American slang doodle (excrement) plus squat, used in the sense of defecating. Doodly-squat was originally the more common form, but diddly-squat overtook it in the early 1980s, and is now an order of magnitude more common in print.
The following explanation is offered by Why You Say It: The Fascinating Stories Behind over 600 Everyday Words and Phrases By Webb Garrison (Rutledge Hill Press, Nashville, Tenn., 1992): “Diddly-squat: Strictly American in origin, that expression sounds suspiciously like a pair of modified barnyard terms — but isn’t. Carneys (carnival workers) who traveled from town to town working one county fair after another developed their own private language. They had to do so in order to attract potential gamblers who would pay for a chance at a gimcrack prize. ‘Diddle-e-squat’ seems to have entered the carnival talk to name money — often a nickel or a dime, since that was the going rate for a game of chance. Frequently used to hide talk about a small amount of money, it was an easy and natural transition for the carnival term to indicate very little of anything.”
So, diddly means the same thing as squat and diddly-squat is just doubling up on the information. It’s one of the formations that makes the language both weird and interesting at the same time.
Thanks to Charlotte for suggesting it.