The full detail (from the OED):
"Etymology: Old English dún (feminine), hill = Old Dutch dúna (Middle Dutch dúne, Dutch duin, whence modern Low German düne sandhill, French dune). Supposed to be of Celtic origin: compare Old Irish dún hill, hill-fort, Welsh din, and place-names in -dūnum.
Since dúna must have been in use at an early date in the West Germanic dialects of Batavia and Lower Saxony, it is doubtful whether the word was brought by the Saxons from the continent, or adopted, after their settlement here, from the Britons; the former alternative is favoured by the exact correspondence in form and gender of the Old English and Old Dutch words, and by the fact that in local nomenclature Old English dún seems to have been confined to the Saxon area. It is, however, in English only that the word has given rise to an adverb and a preposition"