without a shadow of a doubt

without the/a shadow of (a) doubt

With certainty; for sure. I only made such a big change in my life because I knew without the shadow of a doubt that it was the right thing for me to do.
See also: doubt, of, shadow, without
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

without a shadow of a doubt

 and beyond the shadow of a doubt
without the smallest amount of doubt. I am certain that I am right, without a shadow of a doubt. I felt the man was guilty beyond the shadow of a doubt.
See also: doubt, of, shadow, without
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
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References in periodicals archive ?
You'll find out, without a shadow of a doubt, who's worthy of having in your world.