Make no mistake about it!

make no mistake about it

What I have said or am about to say is absolutely certain; do not think otherwise. I intend to take this all the way to the Supreme Court if I have to. Make no mistake about it. Make no mistake about it, this was a carefully orchestrated crime—not some amateur job.
See also: make, mistake, no
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

Make no mistake (about it)!

sent. an expression signifying the sincerity of the speaker’s previous statements. Make no mistake! This is the real thing.
See also: make, mistake, no
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.
See also: