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.
Is There Such A Thing As A True Synonym?Thesauruses present a catalog of synonyms from which we may pick and choose words to put into our writing, but a thesaurus used alone is a very bad thing because there is no such thing as a true synonym. Every word has its particular place in the language.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.