Strong’s Greek
G731 — arrhetos
The entry below is Strong’s own definition (1890, public domain), and every verse where the word occurs.
ἄρρητος
arrhetos — pronounced ar'-hray-tos
from 1 (as a negative particle) and the same as 4490; unsaid, i.e. (by implication) inexpressible:--unspeakable. see GREEK for 1 see GREEK for 4490
1 occurrence in the KJV.
2 Corinthians 12:4
How that he was caught up into paradise, and heard unspeakable words, which it is not lawful for a man to utter.