Literally, "woe to the vanquished." The most (in)famous usage of this phrase came during negotiations between the rather vanquished Romans and the triumphant Gauls during the latter's successful invasion of the Eternal City. On this matter, Livy tells us:
About this time an armistice was agreed to and the commanders allowed the troops to communicate with each other. Gallic soldiers used frequently in talking to tell the Romans that they knew they were starving and ought therefore to surrender, and the story goes that the Romans, to make them believe that they were not, threw loaves of bread from various points their lines down into the Gallic outposts. None the less the time soon came when hunger could no longer be either concealed or endured. Camillus (1) was raising troops at Ardea, where after instructing his Master of Horse (2), Lucius Valerius, to bring up his men from Veii (3), he was busy training a force fit to deal with the Gauls an equal terms -- while the beleaguered army on the Capitol waited and hoped. It was a terrible time: ordinary military duties by now almost beyond their strength; they had survived all other ills that flesh is heir to, but one enemy -- famine -- which nature herself has made invincible, remained. Day after day they looked to see if help from Camillus was near; but at last when hope as well as food began to fail, and they were too weak to carry the weight of their equipment when they went on duty, they admitted that they must either surrender, or buy the enemy off on the best terms they could get -- for the Gauls were already letting it be known pretty clearly that they would accept no very great sum to abandon the siege (4). The Senate accordingly met, and the military tribunes were authorized to arrange the terms; Quintius Sulpicius conferred with the Gallic chieftain Brennus and together they agreed upon the price, one thousand pounds' weight of gold -- the price of a nation soon to rule the world. Insult was added to what was already sufficiently disgraceful, for the weights which the Gauls brought for weighing the metal were heavier than standard, and when the Roman commander objected the insolent barbarian flung his sword into the scale, saying 'Woe to the vanquished!' (5) -- words intolerable to Roman ears.
Nevertheless it was neither God's purpose nor man's that the Romans -- of all people -- should owe their lives to a cash payment. The argument about the weights had unduly protracted the weighing-out of the gold, and it so happened that before it was finished and the infamous bargain completed, Camillus himself appeared upon the scene. (6)
Taken from the 1971 edition of The Early History of Rome, translated from the original Latin by Aubrey de Sélincourt, with commentary added by Robert Ogilive, and edited by Betty Radice.
1: Marcus Furius Camillus, Dictator of Rome at the time of the Gallic siege.
2: This title is hard to describe. Although the full responsibilities of the Master of Horse have been more or less lost to antiquity, it's important to make a distinction between Masters under the Consuls and Masters under the Dictators. Consuls were the 'business as usual' leaders of Rome during the Republic. Every year, two men would be elected to jointly hold executive power. This was incredibly significant since the ancient Romans based their system of dating around consulships. Although it was not terribly efficient after a certain point (what the hell does "in the third consulship of Gaius and the first of Valerius" even mean 200 years after the fact?), that's the way it was done at the time. The consuls would appoint a Master of Horse, and though the full extent of this man's authority is not known, it is accepted that he served as a military advisor of sorts, acting like a Secretary of State, Secretary of Defense, and National Security Advisor all rolled into one. They would advise the consuls during military expeditions. Under the dictators, however, Masters of Horse enjoyed greater authority and greater responsibility. This is directly related to the role of the dictator in the Roman Republic. In times of great national crisis -- and I think a successful invasion by the Gauls qualifies as one -- the Senate would appoint one man to whom the entire nation answered, and to make it even better, there were basically no moral or legal restraints upon the dictator when he was, uh, dictating. For example, if you were sitting around reading Homer and the dictator saw you and said "Cassius, put that goddamned book away and take this sword," you would do it. You wouldn't say "I'm sorry, I don't believe in violence." You know why? Because if you did that, the dictator would reply "oh, okay," and then run you through with a spear. This concept is highly important to fascism, in that it places authority and responsibility on the same axis. The theory goes that one man who is both ultimately authoritative and responsible for the fate of an entire nation will make all the necessary decisions to achieve victory and will be entirely ready, willing, and able to back up his decisions through whatever means necessary. Some modern authors are relatively obsessed with this idea, but I digress. Generally speaking, the man who put forward the name of a potential Dictator in the Senate would be appointed his Master of Horse, and in this capacity, the Master would take on a military command of his own as well as serving as the Dictator's advisor. The Master of Horse in this sense was answerable only to the Dictator and therefore enjoyed much of the same power as his only immediate superior. Though nobody knows for sure, it's believed that the Master of Horse once referred to the commander of the entire cavalry but for whatever reason evolved into an advisor of sorts.
3: Veii was an Etruscan town that for as long as anyone could remember had been a traditional enemy of the fledgling republic. The main reason for this was the fact that after the Romans -- led by Lucius Junius Brutus -- had expelled the Etruscan aristocracy (including their final King, Tarquinius Superbus), the displaced nobles went to all the Etruscan towns to stir up hatred against the Roman upstarts. This worked, generally, since Rome's rapid expansion was understandably viewed with quite a bit of fear and trepidation. What was stopping Rome from going out and dethroning all the Kings of Etruria and claiming all of Italy as its own? (How prescient these Etruscans were!) Veii was located about 21 kilometers to the north-west of Rome and according to some sources, rivalled even Athens in size and population. After countless wars between the two cities, Veii was finally defeated around 396 BC and made a Roman "ally." By this, I mean the Roman soldiers slaughtered or expelled basically all of the city's inhabitants and Roman citizens moved in. Not surprisingly, Veii was now a bit more receptive to Rome's expansion.
4: This is significant because the Gallic invasion of Rome was based on a "my honor's dick is bigger than your honor's dick" series of events that began when the Gauls attacked an ally of Rome and Rome sent a delegation to the Gauls, demanding that they stop. This wouldn't have been so bad if the Romans hadn't tried to use the word "Rome" as their primary deterrent against the Gauls. The Gauls were suitably unimpressed and after a bit of back and forth, they let the Romans have it. So unlike most invasions of the time -- and still many of this time -- the objective was not so much to make a smash and grab for the enemy's resources as it was of making Rome putting its collective foot in its mouth.
5: In modern terms, I think it would be more like "aww, it looks like someone needs to take a ride on the WHAAAAAAAAAAAAAAAAAAAAAAAAAmbulance!"
6: I suppose it goes without saying at this point that Camillus heroically rallied the Romans against the Gauls and they defeated them and cast them out of the city and blah blah blah.
And just for fun, here's an approximate pronunciation: