Definition: A victory that was achieved at too high a cost.
Origin: Pyrrhus (219-272 BC), said to be a direct descendent of Achilles, was king of Epirus in Greece who was one of the greatest generals in the Greek army. Pyrrhus and his troops marched to Rome, destroying the well-trained Roman armies in two major battles in southern Italy. The troops suffered too many casualties and were too weak to storm the city walls, so they eventually had to retreat.
For further reading: Opening Pandora’s Box: Phrases Borrowed from the Classics and the Stories Behind Them by Ferdie Addis, Reader’s Digest (2011).