An Etymological Dictionary of the German Language/Annotated/rösten
rösten (1.), see under Rost (1).
rosten (2.), verb, ‘to steep, water-rot flax or hemp,’ from Middle High German rœȥen, rœtzen, ‘to rot, cause to rot,’ implying a connection with rösten (1); allied to rôȥ, adjective, ‘mellow, soft,’ rôȥȥen, ‘to rot,’ and Old High German rôȥȥên, ‘to rot.’ From a Teutonic root raut, ‘to rot’; compare Dutch rot, ‘rotten, decayed,’ Old Saxon rôtôn, ‘to rot,’ Anglo-Saxon rotian, English to rot, to ret (from Anglo-Saxon *reátian?), ‘to steep, water-rot,’ Old Icelandic rotenn, ‘putrefied’; see rotten (2).