Appendix N — The Word “Donjon”
Professor Skeat and The New English Dictionary derive this word from the Low Latin, dominionem, acc. of dominio, lordship. Leland frequently speaks of the keep as the dungeon, which of course is the same word. Its modern use for a subterranean prison seems to have arisen when the keeps were abandoned for more spacious and comfortable habitations by the noble owners, and were chiefly used as prisons. The word dunio, which, as we have seen, Lambert of Ardres used for a motte, probably comes from a different root, cognate with the Anglo-Saxon dun, a hill, and used in Flanders for the numerous sandhills of that coast.