The name Brecknock is an anglicized form of Brycheiniog, the Welsh name of the territory of Brychan (whence the alternative form of Brecon), a Goidelic chieftain, who gained possession of the Usk valley in the 5th century.
On the departure of the Romans, the Goidelic hill-tribes, probably with help from Gower and Ireland, seem to have regained possession of the Usk valley under the leadership of a chieftain of their own race, Brychan, who became the ancestor of one of the three chief tribes of hereditary Welsh saints.
Cil, a retreat, said to be akin to the Goidelic kil - Ciliau-Aeron, Cilcennin.
Primitive Celtic split up, as already shown, into two dialects, represented in modern times by two groups of languages - (i) the Goidelic group, comprising Irish, Scottish, Gaelic and Manx.
In the Goidelic group qu appears as c, thus Irish cethir, " four "; in the Brythonic group it is changed into p, as in Welsh pedwar, " four."