Twain
Old English twegen, masculine of twā (see two).
wiktionary
From Middle English tweyne, tweien, twaine, from Old English twēġenm(“two”), from Proto-West Germanic *twai-, from Proto-Germanic *twai, from Proto-Indo-European *dwóh₁. Cognate with Saterland Frisian twäin, Low German twene, German zween, Swedish tvenne . 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 is 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.
From Middle English twaynen, from twayne(“two”, numeral) (see Etymology 1 above).
etymonline
twain (n.)
Old English twegen "two" (masc. nominative and accusative), from Proto-Germanic *twa- "two," from PIE root *dwo- "two." It corresponds to Old Frisian twene, Dutch twee, Old High German zwene, Danish tvende. The word outlasted the breakdown of gender in Middle English and survived as a secondary form of two, especially in cases where the numeral follows a noun. Its continuation into modern times was aided by its use in KJV and the Marriage Service, in poetry (where it is a useful rhyme word), and in oral use where it is necessary to be clear that two and not to or too is meant. In U.S. nautical use as "a depth of two fathoms" from 1799.
