Polybius (ca. 203–120 B.C.), describing the naked tactics used by the Gaesatae (a group of Celtic warriors who lived "among the Alps and near the Rhone") against a Roman army at the Battle of Telamon in 225 B.C.:
"The Insubres and Boii wore their trousers and light cloaks, but the Gaesatae had discarded these garments owing to their proud confidence in themselves, and stood naked, with nothing but their arms, in front of the whole army, thinking that thus they would be more efficient, as some of the ground was overgrown with brambles which would catch in their clothes and impede the use of their weapons. At first the battle was confined to the hill, all the armies gazing on it, so great were the numbers of cavalry from each host combating there pell-mell. In this action Gaius the Consul fell in the mellay fighting with desperate courage, and his head was brought to the Celtic kings; but the Roman cavalry, after a stubborn struggle, at length overmastered the enemy and gained possession of the hill. The infantry were now close upon each other, and the spectacle was a strange and marvellous one, not only to those actually present at the battle, but to all who could afterwards picture it to themselves from the reports. For in the first place, as the battle was between three armies, it is evident that the appearance and the movements of the forces marshalled against each other must have been in the highest degree strange and unusual. Again, it must have been to all present, and still is to us, a matter of doubt whether the Celts, with the enemy advancing on them from both sides, were more dangerously situated, or, on the contrary, more effectively, since at one and the same time they were fighting against both their enemies and were protecting themselves in the rear from both, while, above all, they were absolutely cut off from retreat or any prospect of escape in the case of defeat, this being the peculiarity of this two-faced formation. The Romans, however, were on the one hand encouraged by having caught the enemy between their two armies, but on the other they were terrified by the fine order of the Celtic host and the dreadful din, for there were innumerable horn-blowers and trumpeters, and, as the whole army were shouting their war-cries at the same time, there was such a tumult of sound that it seemed that not only the trumpets and the soldiers but all the country round had got a voice and caught up the cry. Very terrifying too were the appearance and the gestures of the naked warriors in front, all in the prime of life, and finely built men, and all in the leading companies richly adorned with gold torques and armlets. The sight of them indeed dismayed the Romans, but at the same time the prospect of winning such spoils made them twice as keen for the fight. But when the javelineers advanced, as is their usage, from the ranks of the Roman legions and began to hurl their javelins in well-aimed volleys, the Celts in the rear ranks indeed were well protected by their trousers and cloaks, but it fell out far otherwise than they had expected with the naked men in front, and they found themselves in a very difficult and helpless predicament. For the Gaulish shield does not cover the whole body; so that their nakedness was a disadvantage, and the bigger they were the better chance had the missiles of going home. At length, unable to drive off the javelineers owing to the distance and the hail of javelins, and reduced to the utmost distress and perplexity, some of them, in their impotent rage, rushed wildly on the enemy and sacrificed their lives, while others, retreating step by step on the ranks of their comrades, threw them into disorder by their display of faint-heartedness. Thus was the spirit of the Gaesatae broken down by the javelineers; but the main body of the Insubres, Boii, and Taurisci, once the javelineers had withdrawn into the ranks and the Roman maniples attacked them, met the enemy and kept up a stubborn hand-to‑hand combat. For, though being almost cut to pieces, they held their ground, equal to their foes in courage, and inferior only, as a force and individually, in their arms. The Roman shields, it should be added, were far more serviceable for defence and their swords for attack, the Gaulish sword being only good for a cut and not for a thrust. But finally, attacked from higher ground and on their flank by the Roman cavalry, which rode down the hill and charged them vigorously, the Celtic infantry were cut to pieces where they stood, their cavalry taking to flight.
About forty thousand Celts were slain and at least ten thousand taken prisoners, among them the king Concolitanus."
Source: The Histories of Polybius, published in Vol. I of the Loeb Classical Library edition, 1922-1927. Translation by W. R. Paton.
I might add that the Alps near the Rhône is where I happen to make my home, but I can't say I've seen any naked warriors, with or without jewelry, in this neck of the woods.