Not know beans

verb phrase

(Variations: diddly or diddley or diddly-damn or diddly-poo or diddly-poop or diddly-shitor diddly-squat or diddly-squirt or diddlywhoop or shit or squat or zilch or zip may replace beans) To be very ignorant; not know even the fundamentals •In each case the positive and negative idiom have the same meaning: You don’t know beans, do you?/ may have been England’s greatest mathematical puzzle inventor, but he knew beans about spiders and flies

[1833+; entry form fr an old joke question, found by 1830, ”How many blue beans make five white?”]
Also, not know the first thing; not know from nothing. Be ignorant about something, as in a poem published in the Yale Literary Magazine in 1855: “When our recent Tutor is heard to speak, This truth one certainly gleans, Whatever he knows of Euclid and Greek, In Latin he don’t know beans.” The beans in this colloquial phrase, dating from the early 1800s, signify something small and worthless; not knowing the first thing about something clearly shows one doesn’t know anything about it at all; and the third slangy phrase, with its double negative, implies stupidity as well as ignorance, as in Poor girl, just starting out and she doesn’t know from nothing.