An Etymological Dictionary of the German Language/Rain
Rain, m., ‘strip or belt of grass as a dividing line between fields, ridge,’ from MidHG. and OHG. rein, m., ‘ridge’ (as a line of division between fields); corresponding to LG. reen, ‘field boundary,’ OIc. rein, f., ‘strip of land.’ Perhaps cognate with Sans. rêkhâ, f., ‘row, line, strip.’