An Etymological Dictionary of the German Language/Brunn
Brunn, Brunnen, Born, m., ‘fountain, spring, well.’ The form with the metathesis of the r is LG.; the first two are based upon MidHG. brunne, m., ‘spring, spring-water, well’; OHG. brunno (beside which a form pfuzzi, ‘well,’ from Lat. puteus, appears in OHG.; comp. Pfütze). It is based upon an OTeut. word; Goth. brunna, ‘spring,’ AS. burna (for brunna), E. bourn (‘brook’). Brunnen has been derived from brennen, for which a primary meaning ‘to heave, seethe’ (comp. MidHG. LG. sôt, ‘well, draw-well’) is assumed without proof. Gr. φρέαρ, ‘well,’ scarcely points to a root bhru, ‘to heave, bubble’ (cognate with brauen?); nn may be a suffix, as perhaps in ModHG. Sonne.