"Padawan" is one of those SW terms, like "Sith," that's been around for ages and ages (specifically, since the 1974 rough draft) even though it's never actually mentioned in the OT.

Which probably means that somebody must have suggested to Lucas at one point that it sounded a bit silly. Rather like the story about Brian de Palma comparing "the Force of Others" (as it was called at the time) to "the Farts of Others." And Lucas, not yet having risen to a position where no one could tell him 'no,' acquiesced--for the moment.