Welsh Expression.
Nadolig comes from old latin which means
birthday.
since the welsh for birthday is Penblwydd, this use of 'birthday' is a specific birthday, the birthday of Christ. It casually means
Christmas, but of course the word
Christmas doesn't literally mean birthday at all.
Llawen means joyful or merry, so Nadolig Llawen is literally merry birthday, and casually means merry xmas.