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not know beans

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.

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The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.