Word Origin & History
there are several theories, all similar, and deriving the word from the tendency to say "both the." One is that it is O.E. begen (masc.) "both" (from P.Gmc. *ba, from PIE *bho "both") + -þ extended base. Another traces it to the P.Gmc. formula represented in O.E. by ba þa "both these," from
ba (feminine nominative and accusative of begen) + þa, nominative and accusative plural of se "that." A third traces it to O.N. baðir "both," from *bai thaiz "both the," from P.Gmc. *thaiz, third person plural pronoun. Cf. O.Fris. bethe, Du. beide, O.H.G. beide, Ger. beide, Goth. bajoþs.