mustn't

must +‎ -n't

• IPA^(key): /ˈmʌs.n̩t/

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mustn't

1. must not (negative auxiliary[1]); used to indicate that something is forbidden or, in a tag question, that something is not necessary.

   You mustn't blame yourself for this tragedy.

   We must go, mustn't we?

   • 1906, Edith Nesbit, The Railway Children, Chapter 4: The engine-burglar,

     "Now, listen," said Mother; "it's quite true that we're poor, but we have enough to live on. You mustn't go telling everyone about our affairs--it's not right. And you must never, never, never ask strangers to give you things. Now always remember that--won't you?"

   • 1989: Batman (movie)

     Look, we mustn't mistake ourselves for regular people. We're artists.

• shouldn't
• can't
• oughtn't

• Appendix:English modal verbs
• Appendix:English tag questions