A dialectal or archaic contraction of 'need not' or 'needs not'—an old English auxiliary verb form.
From Old English 'neodian,' though 'needn't' (the modern contraction) is actually more common; this represents archaic verb conjugation where verbs took different endings based on tense and person.
Verbs like 'needn,' 'mustn,' and 'shan't' are remnants of Old English grammar where auxiliary verbs conjugated like regular verbs—you can actually see the evolution of English grammar frozen in these old-fashioned contractions.
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