smart as a whip
Very intelligent or clever, as in Little Brian is smart as a whip; he's only three and already learning to read. This simile alludes to the sharp crack of a whip. [Mid-1900s] Also see mind like a steel trap.
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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.