Yes, this of-repeated tale was reported by the Romans all the time, but it sounds a lot like rumor. Stuff the victors write about their former enemies, "they were so stupid, they went into battle naked" etc. As historians, we need to look at history with a critical eye and should seek verification from multiple sources before making an firm conclusions.

So, are there any non-Roman sources that mention Celts going into battle naked? Any Celtic records or oral histories, artistic depictions, etc?