mustn't /ˈmʌsnt/ is the contraction of must not. The apostrophe replaces the o, and the transcription shows a written /t/ at the end of the form.
You mustn't touch that wire.
You mustn't leave the gate open. A natural warning, with the contraction spoken smoothly as /ˈmʌsnt/ or /ˈmʌs.ənt/.
You mustn't tell anyone about this. Here the contraction carries a natural warning tone, and it flows smoothly into the next word when spoken.
Mustn't is especially British in everyday use. American English speakers often avoid it, and an American learner dictionary labels it rare and very formal.
The spelling keeps the final t of must not, and the dictionary transcription /ˈmʌsnt/ still shows a /t/, so do not drop that letter when you write it, even though it may sound very light in speech.