My apologies, if you are, but I'm guessing you're not a German speaker..

Follow the German Du/Sie via the Dutch Dank u/Dank je, allow a certain amount of alteration over time (flutterby/butterfly) and it’s obvious where the Du/Th(o)u comes from, along with the Thou/You — regional variation over time means the informal ‘je’ of the Dutch dialect of German … impacted by who knows what Scandinavian influences (not my area of speciality, but Charles Stross could tell you a thing or two about it) … became the formal variation of the Anglo-Saxon English dialect, just as the informal ‘Du’ of German became the formal ‘u’ of Dutch.

Whilst significant, not everything was changed by the arrival of William ̶D̶e̶s̶c̶e̶n̶d̶a̶n̶t̶ ̶o̶f̶ ̶D̶a̶n̶e̶s̶ ̶i̶n̶ ̶N̶o̶r̶t̶h̶e̶r̶n̶ ̶F̶r̶a̶n̶c̶e̶ of Normandy.