mistake for
Take someone or something for someone or something else, as in I'm sorry, I mistook you for her sister, or Don't mistake that friendly smile for good intentions; he's a tough competitor. [c. 1600]
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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.