An Etymological Dictionary of the German Language/Eid
Eid, m., ‘oath, execration,’ from the equiv. MidHG. eit(d), OHG. eid, m.; a word common to Teut., but not found in the other groups; Goth. aiþs, OIc. eiðr, AS. âþ, E. oath, Du. eed, OSax. éth, m.; for the common Teut. aiþa-z, from pre-Teut. ói-to-s (comp. OIr. oeth, ‘oath’), no suitable cognate has yet been found. Ehe and its cognates are scarcely allied to it, though Eidam may be so.