Literally translates as Long Live Wales in the Welsh language, a language that is of Gaelic origin and is virtually extinct in all but rural areas of Wales. Usually shouted after a victory in the sport of rugby, it is a phrase that shows patriotism and nationalism.
Brian: Hey, Wales won the Rugby!
Huw: I know mate, Cymru am Byth!
Brian: Speak english or nothing at all welshie! Also, you truly are all inbred and have contributed nothing to the world!
Huw: Cymru am Byth mate, Cymru am Byth!