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Phrase(s): A little knowledge is a dangerous thing. [and] A little learning is a dangerous thing.
Prov. Cliché If you only know a little about something, you may feel you are qualified to make judgments when, in fact, you are not. • After Bill read one book on the history of Venezuela, he felt he was an authority on the subject, but he wound up looking like a fool in discussions with people who knew a lot more about it than he did. A little learning is a dangerous thing.
McGrawhill's American Idioms And Phrasal Verbs