As an aside "braf" comes from the English "brave"; I've never quite gotten my head around the connection between something brave and something nice :P GPC on-line also notes that it doesn't mutate except for some specific areas in South Wales; I've never heard anyone say "Dwi'n fraf, diolch"! Ha.
As well as meaning "courageous, "brave" often had the meaning of "fine, splendid" in older English, e.g. Shakespeare's "brave new world". This meaning, which was imported into Welsh, is still found in some expressions in modern English. It is also quite a common meaning in modern French. As well as in the Scottish "braw". The dictionaries say this word came into French from old Spanish or Italian. I don't know when and where the extra meaning of "fine, excellent, etc." developed. It certainly originally meant "wild" (hence "courageous") from Latin "barbarus".