In/under no circumstances
Definition 1 of 1
Pro Tip 1/3Emphasis on Prohibition
The phrase adds strong emphasis to rules or prohibitions, indicating something must be completely avoided. 
Under no circumstances are visitors allowed to touch the artwork.
Pro Tip 2/3Fixed Position
The phrase usually comes at the beginning of a sentence for extra emphasis, followed by an auxiliary verb and the subject. 
In no circumstances will we accept late submissions.
Pro Tip 3/3Negative Situations
It's often used to refer to negative situations or outcomes that should be avoided. 
Under no circumstances should you share your password with others.