G0731
ἄρρητος
inexpressible
ἄρρητος, -ον (ἀ- neg., ῥητός, ῥέω), [in Sm.: Lev.18:23 * ;] __1. unspoken (Hom., al.). __2. unspeakable (Hat., al.; frequently in Inscr.; MM, see word): 2Co.12:4.† (AS)
Full Thayer's lexicon entry
ἄῤῥητος — árrhētos
ἄρρητος, ἀρρητον (ῤητός, from Ρ᾽ΑΩ);
a. unsaid, unspoken: Homer, Odyssey 14, 466, and often in Attic.
b. unspeakable (on account of its sacredness) (Herodotus 5, 83, and often in other writings): 2Co 12:4, explained by what follows: ἅ οὐκ ἐξόν ἀνθρώπῳ λαλῆσαι.
1 occurrence