Richard Lenski calls ἄρρητα ῥήματα an "oxymoron," rendering the phrase as "unutterable utterances."
ESV: and he heard things that cannot be told, which man may not utter.
KJV: How that he was caught up into paradise, and heard unspeakable words, which it is not lawful for a man to utter.
NWT 2013: who was caught away into paradise and heard words that cannot be spoken and that are not lawful for a man to say.
Rogers and Rogers: ἄρρητος verbal adj., unspeakable, unutterable. The word was often used of divine secrets not intended for human beings (Windisch; Barrett). ῤῆμα word. ἐξὸν pres. act. part. nom. n. sing. ἔξεστιν it is allowed. The word is to be taken in connection w. ἀνθρώπῳ (s. v. 2); “which it is not lawful for a man to speak” (Plummer).
Zerwick and Grosvenor:
