Well, when you look at the latinate form, it begins with the letters "qu" - words with that in them descend from reconstructed labialized velar stops (kʷ) - and there is precedent for these turning into bilabials, so it's not too far a stretch to see them undergo lenition to a bilabial fricative In fact, it can happen two ways: /kʷ/ > /xʷ/ > /f/ (lenition first) or /kʷ/ > /p/ > /f/ (movement to pure labial first).
I don't know whether this type of phonetic change is actually attested for the Germanic branch of the IE family, but I may go looking if interest remains after I get home from work...
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Date: 2008-01-04 10:10 pm (UTC)I don't know whether this type of phonetic change is actually attested for the Germanic branch of the IE family, but I may go looking if interest remains after I get home from work...