The contraction "needn't" is a shortened form of the verb to "need" and the adverb "not".

The contraction "needn't" functions as a verb or auxiliary verb in a sentence.

Examples:

We needn't rush, we have plenty of time.

You needn't wash the dishes, I'll finish them in the morning.

She needn't have worried. I had the tickets all along.

Girls needn't act helpless around guys.

You needn't speak rudely to your grandparents!

Note that needn't is rarely used today. It is much easier to simply write 'need not' (do or say whatever).