not in a month of Sundays

not in a month of Sundays

Not at any point; under absolutely no circumstances. Not in a month of Sundays did I think that I would actually win the lottery! A: "Do you think Samantha will agree to go on a date with Jake?" B: "Not in a month of Sundays!"
See also: month, not, of, Sundays
Farlex Dictionary of Idioms.

not in a month of Sundays

or

never in a month of Sundays

If you say that something will not or will never happen in a month of Sundays, you mean it is very unlikely to happen. Their scheme will never work — not in a month of Sundays.
See also: month, not, of, Sundays
Collins COBUILD Idioms Dictionary, 3rd ed.
See also:
  • not in a million years
  • never in a month of Sundays
  • never in a million years
  • I think not
  • (someone) thinks (they) are so smart
  • hit the big one
  • go against the flow
  • What are you on?
  • use (one's) head for more than something to keep (one's) ears apart
  • use your head for more than a hatrack