Douglas Harper's Etymology Dictionary
Latin, literally "woe to the vanquished," from Livy, "History" V.xlviii.9.
Wikipedia
Vae victis is Latin for "woe to the vanquished", or "woe to the conquered". Victis is the dative plural form of victus; the dative singular forms of the phrase are vae victo (masculine & neuter) & vae victae (feminine). The phrase serves to remind that those defeated in battle are entirely at the mercy of their conquerors and should not expect—or request—leniency.
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, but claimed that the provided weights were rigged in the Gaul's favor. The Romans complained to Brennus about the issue. Brennus took his sword, threw it onto the weights, and exclaimed, "Vae victis!" The Romans were forced to bring more gold to fulfill their obligation as they then had to counterbalance the sword as well.
Vae Victis is the fifth album by Signal Aout 42.
Usage examples of "vae victis".
He managed a look of unconcern, however, and said, 'Vae victis' and produced two keys from an inner pocket.