Simply that. A language one experiences while in private prayer. I'm not sure I can explain it beyond that.
Praying in a sequence of sounds not normally part of and not as part of a language you know.
AND TRUSTING HOLY SPIRIT IS CONVEYING THE MEANING TO THE FATHER, TO OUR LORD.
Romans 8:26 (The Message)
The Message (MSG)
Copyright © 1993, 1994, 1995, 1996, 2000, 2001, 2002 by Eugene H. Peterson
26-28Meanwhile, the moment we get tired in the waiting, God's Spirit is right alongside helping us along. If we don't know how or what to pray, it doesn't matter. He does our praying in and for us, making prayer out of our wordless sighs, our aching groans. He knows us far better than we know ourselves, knows our pregnant condition, and keeps us present before God. That's why we can be so sure that every detail in our lives of love for God is worked into something good.
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KJV: groanings which cannot be uttered.
perhaps . . . which cannot be put into words, normally.