An Etymological Dictionary of the German Language/Annotated/Brunn
Brunn, Brunnen, Born, masculine, ‘fountain, spring, well.’ The form with the metathesis of the r is Low German; the first two are based upon Middle High German brunne, masculine, ‘spring, spring-water, well’; Old High German brunno (beside which a form pfuzzi, ‘well,’ from Latin puteus, appears in Old High German; compare Pfütze). It is based upon an Old Teutonic word; Gothic brunna, ‘spring,’ Anglo-Saxon burna (for brunna), English bourn (‘brook’). Brunnen has been derived from brennen, for which a primary meaning ‘to heave, seethe’ (compare Middle High German Low German sôt, ‘well, draw-well’) is assumed without proof. Greek φρέαρ, ‘well,’ scarcely points to a root bhru, ‘to heave, bubble’ (cognate with brauen?); nn may be a suffix, as perhaps in Modern High German Sonne.