An Etymological Dictionary of the German Language/Eidam
Eidam, m., ‘son-in-law,’ from MidHG. eidem, m., ‘son-in-law,’ also ‘father-in-law’ (comp. Vetter, Schwager, Base, Neffe, with regard to the fluctuating meaning), OHG. eidum, ‘son-in-law’; corresponds to AS. âðum, OFris. âthum, ‘son-in-law.’ Goth. *aiþmus (?) is wanting, the word mêgs (see Mage) being used. This merely West Teut. term, the derivation of which appears to be similar to that of Oheim, is connected with MidHG. eide, OHG. eidî, Goth. aiþei, ‘mother.’ It is not impossible that it may be allied to Eid also; comp. E. son-in-law. In Suab. and Alem. Eidam is unknown, the word used being Tochtermann.