PIE *pater「父親」から。通常、赤ん坊が発する最も早い音「pa」から来ていると考えられており、語源的にはpope「父性」と同じである。
My heart leaps up when I beholdThe classic example of Grimm's Law, where PIE "p-" becomes Germanic "f-." Spelling with -th- (15c.) reflects widespread phonetic shift in Middle English that turned -der to -ther in many words, perhaps reinforced in this case by Old Norse forms; spelling caught up to pronunciation in 1500s (compare mother (n.), weather (n.)). As a title of various Church dignitaries from c. 1300; meaning "creator, inventor, author" is from mid-14c.; that of "anything that gives rise to something else" is from late 14c. As a respectful title for an older man, recorded from 1550s. Father-figure is from 1954. Fathers "leading men, elders" is from 1580s.
A rainbow in the sky:
So was it when my life began;
So is it now I am a man;
So be it when I shall grow old,
Or let me die!
The Child is father of the Man;
I could wish my days to be
Bound each to each by natural piety.
[Wordsworth, 1802]