In the chazzan's repetition of the Shacharis amida for the first morning of Shevuos, there is a supplication which reads: אז בכתב אשורית, ובלשון עברית, ובדיבור מצרית, הנחלת לבת עברית וכו׳.
What do the words דיבור מצרית mean?
The first two terms are obviously based of the gemara in Sanhedrin 21b, even though the term לשון עברית is confused as עברית was a כתב not a לשון.
But is there reference anywhere to a דיבור מצרית?
Even if it means that at the time of מתן תורהthey were still pronouncing things with an Egyptian accent they picked up over a couple of hundred years, I'm wondering if this point is actually mentioned anywhere.