Lexical Definition

‎ἄῤῥητος‎ árrhētos (ar'-hray-tos) inexpressible ^G:A
from ‎Α‎ (as a negative particle) and the same as ‎ῥητῶς‎

unspeakable.

• unsaid, i.e. (by implication) inexpressible

Abbott-Smith

** ἄρρητος, -ον (< ἀ- neg., ῥητός, ῥέω), [in Sm.: Le 18:23*;]
1. unspoken (Hom., al.).
2. unspeakable (Hat., al.; freq. in Inscr.; MM, s.v.): II Co 12:4.†

Abbott-Smith References

Liddell-Scott-Jones

ἄρρητος, ον,

also η, ον [Refs 5th c.BC+]:— unspoken, ἔπος προέηκεν ὅ πέρ τ᾽ ἄρρητον ἄμεινον[Refs 8th c.BC+]; οὐκ ἐπ᾽ ἀρρήτοις γε τοῖς ἐμοῖς λόγοις not without warning spoken by me, [Refs 5th c.BC+]. adverb -τως, σιγᾶν[Refs 4th c.BC+]

__II that cannot be spoken or expressed, ἀδιανόητον καὶ ἄ. καὶ ἄφθεγκτον καὶ ἄλογον[Refs 5th c.BC+]: hence, unspeakable, immense, [Refs 1st c.BC+]
__III not to be spoken: hence,
__III.1 not to be divulged, ἱροργίαι, ἱρά, [Refs 5th c.BC+]; διδακτά τε ἄρρητά τ᾽, i.e.things profane and sacred, [Refs 5th c.BC+]; ἄ. κόρη the maid whom none may name (i.e. Persephone), [Refs 5th c.BC+]; ἀρρήτων θέσμια, i.e. of Demeter and Persephone, [Refs]
__III.2 unutterable, horrible, δεῖπνα[Refs 5th c.BC+]; ἄρρητ᾽ ἀρρήτων 'deeds without a name', [Refs 5th c.BC+]
__III.3 shameful to be spoken, ῥητόν τ᾽ ἄ. τ᾽ ἔπος[Refs]; ῥητὰ καὶ ἄ. ὀνομάζων 'dicenda tacenda locutus', [Refs 4th c.BC+]. adverb -τως[Refs 3rd c.AD+]
__IV of numbers, ἄρρητα, τά, irrationals, surds, opposed to ῥητά, [Refs 5th c.BC+]