The Lord’s Prayer was recited in the Middle Ages in very much the fashion that one often hears it today, especially by children, with great rapidity and with no shadow of understanding of the words.
The difference is that in the Middle Ages the recitation was in Latin, so that few people knew what they were saying.
Instead of opening with the words, “Our Father,” therefore, it opened with “Pater roster” thus giving rise to the word paternoster as a general name for the Lord’s Prayer.
It was because of this glib and indistinct utterance, then so commonly heard in church, that the slang word patter was formed, taken from the first word of the prayer.