Wiktionary
adv. (context idiomatic English) never ever, not for any reason
Usage examples of "under no circumstances".
The base on Hades must under no circumstances be located or even detected by the Druufs.
I add that under no circumstances will I grant him any cooperation at all and that the hierarchical system is well behind us now.
I exacted the most sacred oaths, that under no circumstances they would bury me until decomposition had so materially advanced as to render farther preservation impossible.
Be very clear about this: should something go wrong with this operation, under no circumstances are you to mount a rescue attempt of any kind.
This was nothing more than a formality instituted to reassure the masses, because as initiates very well knew, during the period of the fast the hunger artist would under no circumstances, including duress, swallow the tiniest morsel.