"Lawn" in Welsh means "full" or "complete," while "da" means "good."
If you're trying to say "good lawn," you would say "caerdydd da" in Welsh, which literally translates to "good lawn."
However, "caerdydd" is not a standard Welsh word for "lawn." "Caerdydd" is the name of the city of Cardiff.
It's possible you've encountered a regional slang term or a mispronunciation.
To get a more accurate translation, please provide more context about what you're trying to express.