Is the word mustn't a pronoun noun or verb?

The Word mustn't is a contraction, a shortened form for must not.

The Word 'must' is an auxiliary verb that indicates obligation, necessity, or likelihood of the main verb; and the Word 'not' is an adverb used to modify the verb as a negative.

Example:

You must not disturb grandma while she's napping.

You mustn't disturb grandma while she's napping.