"the two, the one and the other," there are several theories, all similar, and deriving the word from the tendency to say "both the." One is that it is Old English begen (masc.) "both" (from Proto-Germanic *bai, from PIE *bho "both") + -þ extended base. Another traces it to the Proto-Germanic formula represented in Old English 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 Old Norse baðir "both," from *bai thaiz "both the," from Proto-Germanic *thaiz, third person plural pronoun. Compare similar formation in Old Frisian bethe, Dutch beide, Old High German beide, German beide, Gothic bajoþs.

also ambidexterous, "able to use both hands equally," 1640s, with -ous + Medieval Latin ambidexter, literally "right-handed on both sides," from ambi- "both, on both sides" (see ambi-) + dexter "right-handed" (from PIE root *deks- "right; south"). An earlier English use of ambidexter (adj.) meant "double-dealer, one who takes both sides in a conflict" (late 14c.).

Its opposite, ambilevous "left-handed on both sides," hence "clumsy" (1640s) is rare. Ambidexter as a noun is attested from 1530s (in the sense "one who takes bribes from both sides") and is the earliest form of the word in English; its sense of "one who uses both hands equally well" appears by 1590s.

1650s, with -ity + Medieval Latin ambidexter, literally "right-handed on both sides," from ambi- "both, on both sides" (see ambi-) + dexter "right-handed" (from PIE root *deks- "right; south").

"cutting or working both ways," especially figurative, of arguments, etc., "making both for and against the one using it," 1550s; see double (adj.) + edge (n.).