King Pyrrhus of Epirus fought the Romans at Heraclea in 280 BCE and at Asculum in 279 BCE in what was known as the Pyrrhic War.
Although King Pyrrhus won both battles, he lost so many men that his victories might as well have been defeats. He was fighting in Italy, so the men he lost could not be replaced with new recruits. The Romans, who suffered more casualties than Pyrrhus, had a steady supply of replacement troops. Plutarch quotes Pyrrhus as saying that one more victory like the one at Asculum would “utterly undo him.”
A Pyrrhic victory, therefore, is a technical victory that represents a loss to the victor.