Complex sentence: The first known written use of an ancient Greek transliteration of the suggested P-Celtic term is believed to have appeared within a periplus by the geographer and explorer Pytheas of Massalia, but no copies of this work survive.
Split into simpler sentences: The first known written use of the word was an ancient Greek transliteration of the original P-Celtic term. It is believed to have appeared within a periplus by the geographer and explorer Pytheas of Massalia, but no copies of this work survive.