An Etymological Dictionary of the German Language/rösten
rösten (1.), see under Rost (1).
rosten (2.), vb., ‘to steep, water-rot flax or hemp,’ from MidHG. rœȥen, rœtzen, ‘to rot, cause to rot,’ implying a connection with rösten (1); allied to rôȥ, adj., ‘mellow, soft,’ rôȥȥen, ‘to rot,’ and OHG. rôȥȥên, ‘to rot.’ From a Teut. root raut, ‘to rot’; comp. Du. rot, ‘rotten, decayed,’ OSax. rôtôn, ‘to rot,’ AS. rotian, E. to rot, to ret (from AS. *reátian?), ‘to steep, water-rot,’ OIc. rotenn, ‘putrefied’; see rotten (2).