Vae victis (IPA: [ˈwai ˈwiktiːs]) is Latin for "woe to the vanquished (ones)" or also "woe to the conquered (ones)". (This is the dative plural form—the dative singular is Vae victo if the conquered is masculine, Vae victae if the conquered is feminine.)
In 390 BC, an army of Gauls led by Brennus attacked Rome, capturing all of the city except for the Capitoline Hill, which was successfully held against them. Brennus besieged the hill, and finally the Romans asked to ransom their city. Brennus demanded 1,000 pounds (327 kg) of gold (> €13,250,000), and the Romans agreed to his terms.
Livy, in Ab Urbe Condita (Book 5 Sections 34–49), records that the Gauls provided steelyard balances and weights, and the Romans brought out their gold. But the Romans noticed that the weights were fixed, and the tribunes dared to complain to Brennus about the issue. Brennus took his sword, threw it on to the weights, and exclaimed: "Vae victis!", for the conquered have no rights, forcing the Romans to bring even more gold to fulfill their obligation.