why is this not Sant Dewi? the dictionary says sant is just a noun so it doesn't make sense to me!
In general, it is a way of giving particularly famous or respected historical figures a descriptive title. The second part is often mutated. For example
- Hywel Dda - Hywel the Good
- Rhodri Fawr - Rhodri the Great
- Dafydd Gam - Dafydd (the) Crook-back/Crooked/Lame
- Dewi Sant - Dewi the Saint