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.