Strong's Concordance

Original word: ἀνεκδιήγητος
Transliteration: anekdiégétos
Definition (short): indescribable
Definition (full): inexpressible

NAS Exhaustive Concordance

Word Origin: from alpha (as a neg. prefix) and ekdiégeomai
Definition: inexpressible
NASB Translation: indescribable (1).
NAS Exhaustive Concordance of the Bible with Hebrew-Aramaic and Greek Dictionaries.
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Strong's Exhaustive Concordance

From a (as a negative particle) and a presumed derivative of ekdiegeomai; not expounded in full, i.e. Indescribable -- unspeakable.

see GREEK a

see GREEK ekdiegeomai

KJV: Thanks be unto God for his unspeakable gift.
NASB: Thanks be to God for His indescribable gift!