From the Welsh Brycheiniog, one of the medieval kingdoms of Wales. Named after Bychan Brycheiniog (born 419), the Irish
chieftain who traditionally established a kingdom
there sometime in the 5th century.
Brecknock (pronounced phonetically) was the nearest
the English could get to pronouncing Brycheiniog.
Later shortened to Brecon when it became fashionable
amongst the English not to pronounce consonants. (In
the same way that, for example Brighthelmstone became
Brighton)