Demelza is a Cornish female name which comes from a Cornish place name. The most popular meaning I’ve seen listed for it is “fort of Maeldaf” or “Maeldaf’s fort”, a contracted form of Dinas Maeldaf (dinas being the Cornish word for “fort”) while Maeldaf is an obscure Welsh male name, the first element of the name being “prince”. However, according to British Baby Names and Llewellyn’s Complete Book of Names the origin of that name doesn’t seem likely, considering Maeldaf is an extremely rare Welsh name that doesn’t seem to have had any use outside of Wales, and especially not in Cornwall. The most likely theory they give out is it derives from Cornish elements ty (house) and malsai (eel) essentially meaning “eel house”, so named because it exported eels. It’s also been listed as meaning “thy sweetness” according to the Poldark series of books written by English author Winston Graham, the name of a main character though that doesn’t seem likely.