mustn't

Meanings

1. 1
   contraction

   a contraction of must not, used to express prohibition or a strong negative necessity

   You mustn't open the door until the train stops.

2. 2
   contraction

   used to express a strong negative conclusion or expectation

   She mustn't have heard the warning.

A contraction of must + not. The ending -n't is a reduced form of not, while rapid speech commonly simplifies the consonant cluster so the t of must is weak or absent.

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