Английский Этимологический словарь - father (n.)

- O.E. fжder, from P.Gmc. *fader, from PIE *p@ter, presumably from baby-speak sound like pa. The classic example of Grimm's Law, where PIE "p-" becomes Gmc. "f-." Spelling with -th- reflects widespread phonetic shift in M.E. that turned -der to -ther in many words; spelling caught up to pronunciation in 1500s.