From Old Swedish þrøtter (akin to Old Danish thræt (Danish træt), Old Norse þreyttr), past participle of þrøta (“tire”) (corresponds to Old Norse þreyta and Old English ðrīetan), originally causative of þrȳta (“become tired”), from Old Norse þrjóta, from Proto-Germanic *þreutaną, from Proto-Indo-European *trewd- (“to harass”).