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not know someone from Adam

Be unable to recognize someone, as in Although I have worked here for two months, I've never seen the department head; I wouldn't know her from Adam. This term refers to the biblical story about the world's first human being. As at least one writer has pointed out, differentiating someone from Adam makes little sense since he had no name and wore only a fig leaf. [Mid-1800s]

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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.