An Etymological Dictionary of the German Language/Annotated/schwören
schwören, verb, ‘to swear,’ from the equivalent Middle High German swęrn, swęrjen, Old High German swęren; swęrien; a specifically Teutonic word (like Eid); compare Gothic swaran, Old Icelandic sverja, Anglo-Saxon swęrian, English to swear, Dutch zweren, Old Saxon swęrian, ‘to swear.’ The Teutonic root swar contained in these words had, however, a wider sense originally than the one given, for traces in particular dialects lead us to infer that ‘to answer’ was the meaning of the root; compare Old Icelandic svǫr, neuter plural, ‘answer,’ svara, verb, ‘to answer,’ in the legal sense also ‘to give security,’ andsvar, neuter, ‘legal decision,’ Anglo-Saxon andswaru, feminine, ‘answer,’ English to answer (see Antwort), Old Saxon andswor, ‘vindication.’ The primary idea of the Teutonic root swar is therefore, perhaps, ‘to be responsible’; it has been compared to Latin respondeo from spondeo.