Ok, so the French wikipedia article makes it simple:
And says that it comes from the Greek words 'platís' which means 'flat, large', and 'pois' (which would have given 'pus' in English) which means 'foot', then 'flat foot'.
The French word is made of 'ornitho-rhynkos' which means kinda 'which has a bird beck', ornitho being used in most French words relative to birds, seems then that there is a 'continental school' more interested in the beck, and the Anglos seem to be more interested in his feet...