νη | Abarim Publications Theological Dictionary (New Testament Greek)

There are three separate words νη (ne) in Greek:

The particle of negation νη (ne), which is related to our English word no. This word does not occur independently but always as a prefix, for instance in νηπιος (nepios), speechless, and νηστις (nestis), meaning not having been eating.
The particle of strong affirmation νη (ne), which appears to have originated in an evocation of a deity whose name would follow our particle in the accusative (by Jove! by God!). In rhetoric, our particle serves to refute, object, adjure or contest an antagonist, and may be followed by γαρ (gar), meaning for or because, after which the speaker would explain his certainty. And our speaker may interject our particle into a general statement, to add to its force. This particle occurs in 1 Corinthians 15:31 only, where Paul swears by his audience's loud affirmation.
The noun νη (ne), which is short for νεος (neos), youth. This abbreviated word is not used in the New Testament but was fairly common in the Classics.