From Middle High German brunne, from Old High German brunno, from Proto-West Germanic *brunnō, from Proto-Germanic *brunnô, whence also Old English burna, Old Norse brunnr. The form is Upper German, most Low and Central German dialects using Born (now poetic in the standard language). This southern origin also explains the vocalism. The expected form is obsolete Bronnen with -u- → -o- before a nasal geminate.
- Brunnen in Duden online