in no uncertain terms

Emphatically, definitely so. For example, Jane told them in no uncertain terms that she wanted no part of their practical joke. The double negative in this idiom serves for emphasis. [Mid-1900s] Also see in so many words.

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.