An Etymological Dictionary of the German Language/wenden
wenden, vb., ‘to turn, change,’ from MidHG. węnden, OHG. węnten (from *wantjan), wk. vb., ‘to overturn, cause to retrograde, hinder.’ Comp. OSax. węndian, Goth. wandjan, AS. węndan, ‘to turn,’ E. to wend, Du. wenden, ‘to change’; a factitive of winden. Hence auswendig, ‘by heart,’ MidHG. ûȥwęndic, ‘externally, abroad’ (etwas auswendig können is a ModHG. phrase, and is here used in the sense of ‘externally,’ i.e., ‘without looking into a book’); inwendig, ‘internally,’ from MidHG. innęwendec. —