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 and the Romans agreed to his terms.
Livy, in Ab Urbe Condita (Book 5 Sections 34–49), recorded that the Gauls provided steelyard balances and weights which were used to measure the amount of gold. The Romans brought the gold and noticed that the provided weights were fixed. The Romans complained to Brennus about the issue. Brennus took his sword, threw it on to the weights, and exclaimed, "Vae victis!" The Romans were forced to bring more gold to fulfill their obligation.