Be quite ignorant about something. This colloquialism dates from the early nineteenth century, when beans denoted something small or worthless. However, from the mid-1800s to the early 1900s the converse, to know beans, meant to be quite knowledgeable. The latter has not survived, but we still hear, for example, “Don’t ask Harry’s advice; he doesn’t know beans about cars.” A more specific and much ruder variant is not know one’s ass from one’s elbow, which dates from about 1900 and denotes general stupidity. Other versions replace “elbow” with “hole in the ground,” but “elbow” is heard more often and has become a cliché. A 1963 citation has it, “These medics don’t know their asses from their elbows.”