Father has always been the formal way of referring to your male progenitor, but in recent years both dad and daddy have been gaining in usage while it flatlined well above them. The words dad and daddy were likely fathered by the Middle English words dadd and dadde, both of which have an uncertain origin. It is likely the words have been around in English much longer and simply were not recorded, so some etymologists theorize that it traces to Old English atta, also aetta, possibly from Proto-Germanic atto, meaning "father", and ultimately from Proto-Indo-European atta. However, it is difficult to ascertain whether such a straight line of evolution exists, because similar sounds with a's, d's, and t's also mean "father" all over the European continent. Maybe the word has just been floating around for a while. There certainly has been very little variation in all languages, both phonetically and semantically: tracing it through philology is therefore hard.
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.