ɪt dᵻɹˈaɪvz fɹʌmðə ɡˈɔːlɪʃ stˈɛm pˈɛtɹuː fˈoːɹ ɐtˈætʃt tə kˈɔːɹɪˌoʊz ˈɑːɹmi, ˈæftɚɹ ɐ ɡˈælɪk kˈʌstəm ʌv ɪŋklˈuːdɪŋ nˈʌmbɚz ɪn tɹˈaɪbəl nˈeɪmz ˈiː.dʒˈiː. vˈoʊkəntˈɪaɪ, vˈoʊkˈɔːɹɪˌaɪ, tɹˈaɪkˈɔːɹɪˌaɪ, sˈuːsaɪˈoʊniz.
It derives from the Gaulish stem petru- ('four') attached to corios ('army'), after a Gallic custom of including numbers in tribal names (e.g. Vo-contii, Vo-corii, Tri-corii, Suess-iones).