Is this the radical form? I had a Welsh friend who used to use the word "twp" in his English. Funnily enough, the Russian "tup" means the same.
'Dwp' is the mutated form and you don't ever see it on its own like this which is why this sentence was deleted a while ago by us but, unfortunately still lingers in the system.
'Twp' is the unmutated form and as you suggest it is still used in English in Wales in areas where Welsh was recently spoken as a community language, such as the South Wales valleys. Quite often it is mispronounced as 'tup'
You would use 'dwp' in sentences like:- Mae Owen yn dwp (Owen is stupid); Mae e'n dwp (He is stupid)