An Etymological Dictionary of the German Language/schwören
schwören, vb., ‘to swear,’ from the equiv. MidHG. swęrn, swęrjen, OHG. swęren; swęrien; a specifically Teut. word (like Eid); comp. Goth. swaran, OIc. sverja, AS. swęrian, E. to swear, Du. zweren, OSax. swęrian, ‘to swear.’ The Teut. root swar contained in these words had, however, a wider sense orig. than the one given, for traces in particular dials. lead us to infer that ‘to answer’ was the meaning of the root; comp. OIc. svǫr, n. plur., ‘answer,’ svara, vb., ‘to answer,’ in the legal sense also ‘to give security,’ andsvar, n., ‘legal decision,’ AS. andswaru, f., ‘answer,’ E. to answer (see Antwort), OSax. andswor, ‘vindication.’ The prim. idea of the Teut. root swar is therefore, perhaps, ‘to be responsible’; it has been compared to Lat. respondeo from spondeo.