not know beans about something

Example: “I don't know beans about cars, so I let the mechanic decide.”

Origin: American, from the 1800s, and part of a whole family of expressions in which the bean stands for the smallest quantity worth mentioning — not worth a hill of beans, full of beans, spill the beans. Beans were cheap, common, and individually negligible, which made them the natural unit for measuring almost nothing. There may also be a connection to an older British test of elementary competence, phrased as knowing how many beans make five: a person unable to answer that was understood to know nothing at all, and the American form may be a compressed descendant. Either way the phrase asserts something stronger than ignorance of detail — not knowing beans is not knowing even the minimum that would justify speaking.