In no uncertain terms
Definition 1 of 1
Pro Tip 1/3Emphasizing Certainty
Use when you want to express something with strong conviction, leaving no room for doubt or debate. 
He informed them in no uncertain terms that the deadline was non-negotiable.
Pro Tip 2/3Serious Tone
It often carries a serious or stern tone; avoid using it in light-hearted or casual situations. 
She explained in no uncertain terms the severity of breaking the company's rules.
Pro Tip 3/3Not for Subtlety
Avoid when a situation calls for indirectness or a soft approach; it's for clear-cut communication. 
The coach said in no uncertain terms that only full effort would be accepted at practice.