know whereof (one) speaks

know whereof (one) speaks

formal To be knowledgeable about or familiar with what one is saying. Yes, I know whereof you speak, though I have yet to deal with such a situation myself. Oh, don't pay attention to that old fool. He knows not whereof he speaks.
See also: know, speak, whereof
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

know whereof one speaks

to know well the subject that one is speaking about. You are wrong! You do not know whereof you speak. He simply doesn't know whereof he speaks.
See also: know, one, speak, whereof
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
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