"Dw i'n ofnadwy."
Translation:I am terrible.
Could this also be understood as “I'm awesome”? The Geiriaduc Prifysgol Cymru lists that meaning, too.
That is probably referring a more old-fashioned use of 'awesome', meaning 'terrible, to be feared', etc.
The GPC is a historical and etymological dictionary, so some of the meanings that it gives can be old-fashioned ones, in both Welsh and English usage. Geiriadur yr Academi (English to Welsh only), www.gweiadur.com and the Gomer printed dictionaries may be better guides to current usage.