Vine's Expository Dictionary of NT Words 
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.
denotes "unable to be told out" (eklaleo, "to speak out"), 1—Peter 1:8 , of the believer's joy.
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."
King James Dictionary 
UNSPE'AKABLE, a. That cannot be uttered that cannot be expressed unuterable as unspeakable grief or rage. 2 Corinthians 12 .
Joy unspeakable and full of glory. 1 Peter 1 .
Webster's Dictionary 
(a.) Not speakable; incapable of being uttered or adequately described; inexpressible; unutterable; ineffable; as, unspeakable grief or rage.