Lexical Definition

‎νή‎ nḗ (nay) as surely as ^G:PRT
probably an intensive form of ‎ναί‎

I protest by.

• a particle of attestation (accompanied by the object invoked or appealed to in confirmation)
• as sure as

Abbott-Smith

νή, particle of affirmation employed in oaths, [in LXX: ν. τ. ὑγίειαν, Ge 42:15, 16 (חֵי)*;]
by: c. acc., I Co 15:31.†

Abbott-Smith References

Liddell-Scott-Jones

νή,

Particle of strong affirmation, with accusative of the divinity invoked, once in Trag., ν. τὼ Λαπέρσα, ν. τὸν Εὐρώταν τρίτον, ν. τοὺς ἐν Ἄργει καὶ κατὰ Σπάρτην θεούς [Refs 5th c.BC+]; frequently in Comedy texts and Prose, especially in the phrase νὴ Δία, [Refs 5th c.BC+]; with the Article, ν. τὸν Δία [Refs 4th c.BC+]; with the names of other gods, usually with Article, ν. τὴν Δήμητρα [Refs 5th c.BC+]; ν. τὴν Ἄρτεμιν[Refs 5th c.BC+]; ν. τὼ θεώ (Demeter and Cora) [Refs 5th c.BC+]; ν. τὼ σιώ (of the Dioscuri) used by a Spartan, [Refs 5th c.BC+]; ν. τοὺς θεούς [Refs 5th c.BC+]; ν. τόν alone, [Refs 1st c.AD+]

__II ν. (τὸν) Δία is also used,
__II.1 in answering questions, [Refs 5th c.BC+]
__II.2 in introducing objections or contentions supposed to come from antagonists, followed by γάρ, [Refs]; followed by ἀλλά, [Refs]; preceded by ἀλλά, [Refs 5th c.BC+]
__II.3 to add force by way of climax, ἄλλως τε μέντοι ν. Δία πάντως καὶ.. [Refs 5th c.BC+]
__II.4 in adjurations, [Refs 5th c.BC+]