"Rwyt ti wedi bod yn ofnadwy o ddiog heddiw."

Translation: You have been terribly lazy today.

[MatthewSke1]

I don't think the structure of "yn ofnadwy o" meaning "terribly" was included in the Tips & notes for this section. Can this structure be used to form other adverbs?

[ibisc]

Yes.

[taffyexpat]

I think this should mean "ever so" as well as terribly