The only concrete record of the Cassi is a name on list. The list is of five British peoples who went to over to Caesar's side following the Romans inflicting a heavy defeat on Cassivellaunus. They sent ambassadors who mapped out Cassivellaunus' capital for the Romans. (DBG Book 5) Can we learn or theorise anything else about them?

Some people have suggested that Cassi is an old name for the Catuvellauni. The reason for this is chiefly the name of Cassivellaunus himself. The Catuvellauni are not actually mentioned by Caesar - who in fact never tells us what Cassivellaunus' tribe is, but he does say that his territory is on the North side of the Thames, which is the same as that of the Catuvellauni. Most historians think he was Catuvellaunian, therefore. His name is read as two elements, kassi- wellauno- taken to mean "Champion of the Cassi". I haven't read all the arguments in favour of this, but I can see problems with it. First of all, it would mean that the Cassi, in going over to Caesar's side, were stabbing their own chief in the back and not only that, but plotting with him how best to sack their own capital. It could be, of course, that there had been a sort of Cassi "palace coup" while Cassivellaunus was away fighting and a new clan had the kingship and that Cassivellaunus' seat of power was not theirs... but it's a big "if" and Caesar doesn't mention anything of the kind. Second, I've been through the list on L'Arbre Celtique and personal names with a tribe's name are unknown among Britons and Gauls (with one possible exception, depending on how you read a certain Sequanian coin legend) so "Champion of the Cassi" would be a very weird name. Third is a linguistic argument. This is purely my theory, but I feel quite confident about it (for now).

If we have a Latin plural Cassi, it's pretty much a certainty that the original word would be a Celtic o-stem. The singluar would be *Cassos from a stem *kaso-. The Romans Latinised Celtic (and also Greek) -os endings to -us endings and gave them an -i plural. In British and Gaulish compounds, the first element is normally the stem, without any declension, so we would expect "Champion of the Cassi" to be *Cassovellaunos > Latin *Cassovellaunus and not Cassivellaunus. Incidentally, this means that the British Cassi have nothing to do with the various Gaulish tribes with names ending -casses; their names are from an i-stem, kasi- like that of Cassivellaunus. The stem *kasi- survives in Welsh and Irish and carries the meanings "Adversary" or "Bitter dispute". Cassivellaunus is therefore "The Opposing Champion" or perhaps "He who has Triumphed Over Opposition". What does Cassi mean? There's a possible *kaso- stem in Irish (see EDIL) which has to do with curls, twists, intricacy and difficulty. The Cassi could perhaps be "The Curly-Haired People", "The People on the River Bend" or "The Complex People".

What does it mean if the Cassi aren't the Catuvellauni? Where did they live? The four other tribes are listed as Cenimagni, Segontiaci, Ancalites and Bibroci. Are they neighbours of the Cassi? A popular theory is that "Cenimagni" is really Latin Iceni Magni (the Great Iceni) or Celtic Eceni Magli (the Royal Iceni) and the Cenimagni are therefore the Iceni of East Anglia. Unfortunately, Segontium is the Roman name for Caernarvon on the West Coast of Wales, so if it's anything to do with the Segontiaci, they're a long way away from the Cenimagni, interest in the conflict must have been very widespread and the Cassi therefore could have lived anywhere... except, of course, that if Cassivellaunus was Catuvellaunian, we know that his descendant, Caratacus, fled to Wales which suggests that there might have been an alliance or dynastic links with tribes in Wales, explaining the interest from an apparently remote region. The Cassi might then be expected to be somewhere nearby. If we follow Caesar, we can rule out Kent; it apparently was a confederacy of four rulers, all of whom sided with Cassivellaunus. That leaves the Southern, Western or Northern borders of Catuvellauni territory - the lands of the Atrebates, Dobunni and Corieltauvi/Coritani during the Roman occupation. Where there is a big bend in the Thames, on the South bank, is an Iron Age hillfort near Cassington Mill in Oxfordshire. Cassington is from an Old English word for watercress - caerse - not apparently from the Cassi, but all the same...! Any attempt at a precise location is a guess. Beausale Camp in Warwickshire is another hillfort in a river bend, on the Northwest border of Catuvellaunian territory.

Returning to the Segontiaci, The suffix -iacos is attested in Gaulish as a patronymic (not AFAIK in Brythonic) so the Segontiaci might not be the people from Segontium, but the family of Segontos or Segontios. That would make them more of a sept or clan than a tribe. The five tribes might not be tribes at all, only septs within the Catuvellauni who had decided that since Caesar couldn't be beaten, they would kick out Cassivellaunus and elect a new king. Caesar doesn't say anything on the matter, however. In this case the Cassi could be the Catuvellauni in the sense that they are possible holders of the Catuvellaunian kingship. Or perhaps the Cassi were a sept of the Trinovantes and when Caesar asked for hostages from Mandubracius, he sent some from each of the five septs who made up his tribe, including the Cassi. In this case they could be candidates for the Trinovantian kingship.

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