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- Brennus (4th century BC), chieftain of the Senones, a Gallic tribe originating from the modern areas of France known as Seine-et-Marne, Loiret, and Yonne; in 387 BC, in the Battle of the Allia, he led an army of Cisalpine Gauls in their attack on Rome.
- Brennus (3rd century BC), one of the leaders of the army of Gauls who invaded Macedon and central Greece and defeated the assembled Greeks at Thermopylae.
The recurrence of the name Brennus makes it possible that it was a title rather than a proper name. Some 19th century scholars connected the name with the modern Welsh word for "king", brenin, though Brennus and brenin have since been proven to be unrelated (brenin – earlier breenhin – being derived from the Celtic *brigantinos, meaning "(someone) pre-eminent, outstanding"). As early as the 12th century AD, authors such as Geoffrey of Monmouth (in his Historia Regum Britanniae were connecting the name Brennus with the Welsh personal name Bran meaning "Crow", though the similarity of the names is superficial and are not likely to share a common linguistic origin.
Examples in different forms of the name are:
- Brinno, whose name was said by Tacitus to be that of "a family of rebels".
- The personage named "Brennius" in Geoffrey of Monmouth's Historia Regum Britanniae who conquers Rome, probably created by that author from the two Brenni of history.
- Guest, Edwin; Origines Celticae (1883)
- Karl, Raimund Thoughts on the Evolution of Celtic Societies University of Wales, 2007
- Thomas Healy, Times Literary Supplement 24 June 2005 p 25, reviewing Philip Schwyzer, Literature Nationalism and Memory in Early Modern England and Wales, Cambridge, 2005
- John T. Koch, "Brân, Brennos: an instance of Early Gallo-Brittonic history and mythology'", Cambridge Medieval Celtic Studies 20 (Winter 1990:1-20)