under no circumstances

under no circumstances

At no time; never. Under no circumstances will I allow you to go to a party on a school night.
See also: circumstance, no
Farlex Dictionary of Idioms.

under no circumstances

 and not under any circumstances
Fig. absolutely never. Andy: Under no circumstances will I ever go back there again! Rachel: Why? What happened? Sue: Can I talk you into serving as a referee again? Mary: Heavens, no! Not under any circumstances!
See also: circumstance, no
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

in/under no circumstances

used to emphasize that something should never happen or be allowed: Under no circumstances should you lend Paul any money. Don’t open the door to strangers in any circumstances.
See also: circumstance, no
Farlex Partner Idioms Dictionary

under

/in the circumstances
Given these conditions; such being the case.

under no circumstances

In no case; never.
See also: circumstance, no
American Heritage® Dictionary of the English Language, Fifth Edition.
See also:
  • on one's
  • on someone's
  • out of one's
  • (I've) got to go
  • save someone's skin
  • (Have you) been OK?
  • other than
  • other than (something)
  • pillow-biter
  • let something drop