[NB The brief explanation given below has been drafted on the assumption that the reader is already familiar with the basic building-blocks used in Celtic topographical place-names (Chapter 1 of the Home menu), with the structure of compound place-names (Chapter 2) and with the structure of Celtic river-names (Chapter 19)]
The tribal name is given in the above form by Ptolemy, as is a variant form Damnoni.
This tribal name is clearly based on a topographical place-name of the form Dumnonion, this comprising the inversion-type element dum, meaning ‘summit of hill’, qualified by the hill-letter n. The name simply indicates that a hilltop fortress or settlement which had been occupied by a people who used the hill-letter n was taken over by a people who used the hill-letter m, indeed by the people who came to be called the Dumnoni. The question as to whether the initial element of the name should be Dum or Dam is of no importance. One sees this element as Dem in Ravenna’s Demerosesa.