The Gaesatae (Greek ) were a group of Gaulish warriors who lived in the Alps near the river Rhône and fought against the Roman Republic in the Battle of Telamon of 225 BC. The Greek historian Polybius says their name meant "mercenaries", though the literal meaning in Gaulish is "Armed with javelins, Spearmen"[clarification needed] (from Gaulish *gaison "spear, javelin"), which matches Old Irish gaiscedach "champion, armed person", from gaisced "weapons", itself from gáe "spear, javelin"), and compare quite closely with the medieval Irish fianna, who were small warbands of landless young men operating independently of any kingdom.
According to Polybius' account, the Boii and Insubres of Cisalpine Gaul paid the Gaesatae, under their leaders Concolitanus and Aneroëstes, large sums of money to fight against the Romans, in response to the Roman colonisation of the former Gallic territory of Picenum. The Gauls overran and defeated a Roman army on the approach to Rome, but when the consul Lucius Aemilius Papus arrived with his troops, the Gauls followed Aneroëstes' advice to withdraw with their booty. Papus pursued them, and the other consul Gaius Atilius Regulus cut them off at Telamon in Etruria.
Polybius describes how the Gaesatae fought at the front, and unlike their Gallic allies who fought in trousers and light cloaks, they went into battle naked, both because of their great confidence and their desire not to get their clothes caught in the brambles.Diodorus Siculus also reports that some Gauls fought naked, trusting in the protection of nature. The appearance of these well-built naked warriors, and the noise of their trumpets and war-cries, intimidated the Romans, but their small shields offered little protection against Roman javelins, and the Gaesatae were driven back and their allies slaughtered. Concolitanus was captured. Aneroëstes escaped with a few followers and took his own life. In 222 BC the Gaesatae were hired again, but the Gallic forces were defeated by the Roman cavalry at Clastidium in the territory of the Insubres.
According to Plutarch, in his life of Marcellus, the Gaesatae numbered 30,000 as they crossed the Alps, of whom 10,000 fought at Clastidium.