In a book I'm reading these days, the author mentions the various names of the plough in a few Indo European languages. When he comes to cite the German one (Pflug) he casually adds the far-reaching remark that "the fact that Pflug starts with the letter 'p' is almost certainly the indication of a foreign origin".
Leaving aside the fact that the Celts are often credited for the invention of the wheeled plough fitted with an iron ploughshare (as opposed to the more archaic ard or scratch plough), how reliable is the claim that German words starting with the letter 'p' have a high probability of being loanwords and why would that be the case?