The word is all over the place in Plato and his contemporaries and it’s not in Homer. Philosophy – Wikipedia guesses that Pythagoras probably came up with it first.
The basis for that guess, from what I can tell, is that as cited in LSJ (s.v. φιλόσοφος), both Cicero (Tusc. 5.3.9) and Diogenes Laertes (prooem 12) report that Pythagoras called himself a philosophos “lover of wisdom”, and not just a sophos “wise man, scholar”. That implies to me that this terminology was an innovation by Pythagoras. Pythagoras was certainly early enough in the philosophical tradition for that to make sense.