Reconstruction:Proto-West Germanic/dūnā

• *dūn m, *dūnu f, *dūnō m

Uncertain and disputed; possibly from Proto-Germanic *dūnaz (“accumulation, pile, heap, mound”) (whence also Old Norse dúnn (“pile, heap, assembly”)), from Proto-Germanic base *dū- (“to whisk, whip up, excite, agitate, storm, be stormy”), from Proto-Indo-European *dʰewh₂- (“smoke, dust, haze”). Unlike permanent hills, sand dunes are ever-shifting due to storms and wind, and are not optimal places to build fortifications.
Alternatively, perhaps borrowed from Proto-Celtic *dūnom (“stronghold, rampart”), from the same Proto-Indo-European root above, which also developed the sense of "hill, fortified hill" in Brythonic; however phonology makes a borrowing from Brythonic untenable, as the vowel there shifted from ū to i giving Proto-Brythonic *din where Germanic still clearly has ū.

*dūnā f

1. sand dune
2. hill

• Old English: dūn f or m
  • Middle English: dune, doun
    • English: dune, down
    • Scots: doun, down
• Old Saxon: *dūna f, *dūn m
  • Middle Low German: dûne, dunne f, don, done, donne m
    • German Low German: Düne, Düün
    • → German: Düne
    • → Swedish: dyn, dyne
    • → Norwegian: dyne
    • → Danish: dyne
• Old Dutch: *dūna
  • Middle Dutch: dûne f or n
    • Dutch: duin f
      • Afrikaans: duin
    • → Old Frisian: dūne f, *dūn n (also possibly borrowed from Low German)
      • Saterland Frisian: Dune, Dúune, Duune f
      • West Frisian: dún n
    • → Old French: dune
      • Middle French: dune
        • French: dune
• Old High German: *tūna, dūna f