Don't know if this helps, but I found an older English to Welch dictionary online. Though I could not find the full word, I found each component of the suspected compound noun.
"Goreu" translates to "best" as you can see
here.
And Duw, of course, to "God" as you can see
here. (Click the blue page numbers to get the full entry.)
Not sure if this is related to the expletive discussion, but as you can see with one of the examples listed for "Duw," when the "dd" or "Dd" is used, it is for the expression
Good God! or
Och Dduw!
Of even more interest is the use of the lower case "dd" which seems to be employed in relation to the terms
to deify and
to idolize - or reserved more "gods" than "God" (which uses the upper case - so it probably would have been "Goreu Duw" if employed in a sacred sense?).
This may be wide of the mark, but are we in Dorian Gray territory with this term?