From Middle English tweyne, tweien, twaine, from Old English twēġen (“two”, masc), from Proto-Germanic *twai, from Proto-Indo-European *dwóh₁. Cognate with Low German twene, German zween. More at two.
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.
- (dated) two
- But the warm twilight round us twain will never rise again.
- Bring me these twain cups of wine and water, and let us drink from the one we feel more befitting of this day.
- 1866, Algernon Swinburne, Before Parting, lines 1-2
- A month or twain to live on honeycomb
- Is pleasant;
- 1889, Rudyard Kipling, The Ballad of East and West, line 1
- Oh, East is East and West is West, and never the twain shall meet.
- 1900, Ernest Dowson, Amor Profanus, lines 26-28
- […] all too soon we twain shall tread
- The bitter pastures of the dead:
- Estranged, sad spectres of the night.