Origin of twain
- Middle English tweien, twaine from Old English twēgen dwo- in Indo-European roots
From American Heritage Dictionary of the English Language, 5th Edition
- The word outlasted the breakdown of gender in Middle English and survived as a secondary form of two, then especially in the cases where the numeral follows a noun. Its continuation into modern times was aided by its use in KJV, the Marriage Service, in poetry (where it's commonly used as a rhyme word), and in oral use where it is necessary to be clear that two and not "to" or "too" is meant.
- It could look like one of the many English words inherited from Old Norse. The modern Danish word is "tvende" (pronounced tvenne), it means both, two of a kind, etc.