A derwent derwen is a colloquial term for a person who is or acts stupid or retarded. It refers to a large estate (The Derwent) where people with learning disabilities live and run a garden shop. It is a term which comes traditionally from North Shropshire but is begining to recieve more mainstream usage.
"Dont be such a derwent"
"Stop acting like you live at the derwent"
"Your such a derwent-face"