"Dw i'n meddwl mod i'n barod."

Translation: I think that I am ready.

[JamesTWils]

Why mod instead of bod?

[EllisVaughan]

It's because the traditional sentence is "Dw i'n meddwl fy mod i'n barod" but the fy has been dropped.

[mizinamo]

....but the nasal mutation caused by fy has remained.