I’m not sure what the source is, but googling the phrase leads me to just one writer who says it is spurious in origin. The meaning is “As Petra [verb] Petro, Peter crowns Rudolpho”.
This is an allusion to Matt 16:18 - “You are Petros, and upon this Petra I shall build my church.” Protestants say Petros means small stone or pebble, but Petra means huge rock - i.e., Peter was not the Rock, because the Rock was Christ.
If a Pope really wrote those words in order to crown Rudolph as king, then he basically meant that just as the large rock (christ) instituted the small pebble (Peter), so too does the Pope crown Rudoph as king.
I’ve got lots of problems with this, and a few replies. But what I need to know as a starting point is whether these words were indeed written, and if so, what precisely the Pope meant.
The problem is a word gender issue. In Greek, many words have gender forms for male and female. It could not have been petra, since that is the feminine and would have been an insult to rename Simon bar Jonah to a female name. Since Our Lord spoke Aramaic or Hebrew, He would have said Cephas (Rock) which would be translated “Thou art Rock and upon this rock I will build my Church.” It is only after it was translated into Greek that the word gender issue comes up.