Unspeakable - Vine's Expository Dictionary of New Testament Words
Unspeakable[ 1,,G411, anekdiegetos ]
denotes inexpressible" (a, negative, n, euphonic, ekdiegeomai, "to declare, relate"), 2 Corinthians 9:15, "unspeakable" (of the gift of God); regarding the various explanations of the gift, it seems most suitable to view it as the gift of His Son.
[ 2,,G412, aneklaletos ]
denotes "unable to be told out" (eklaleo, "to speak out"), 1 Peter 1:8, of the believer's joy.
[ 3,,G731, arrhetos ]
primarily, "unspoken" (a, negative, rhetos, "spoken"), denotes "unspeakable," 2 Corinthians 12:4, of the words heard by Paul when caught up into paradise. The word is common in sacred inscriptions especially in connection with the Greek Mysteries; hence Moulton and Milligan suggest the meaning "words too sacred to be uttered."