An Etymological Dictionary of the German Language/Annotated/Ader

Ader, feminine, ‘vein,’ from Middle High German âder, Old High German âdara, feminine, ‘vein, sinew,’ corresponding to Middle Low German ader, ‘vein, sinew,’ Dutch ader, Anglo-Saxon œ̂dre, feminine, ‘vein’ (rarely êðr), Old Swedish aþra, Modern Swedish ådra; also without the derivative r, Old Icelandic œ̂ðr (the r is simply a nominative suffix), feminine, ‘vein;’ the Gothic cognate êþ is not found. The pre-Teutonic êt- has been connected with Greek ἥτορ, ‘heart,’ ἦτρον, ‘abdomen,’ and here it must be recollected that Middle High German and Middle Low German âder in the plural may signify ‘bowels.’