mustn't
uk/ˈmʌs.ənt/us/ˈmʌs.ənt/short form of must not:
You mustn't worry too much about this.
More examples
- You mustn't go into the garden.
- Tell them they mustn't make too much noise.
- You mustn't leave litter lying around.
- I mustn't forget my keys.
- She mustn't feel she has to pay.