Twain means two, in old use. In twain, cut it in twain, never the twain shall meet.
It survives in two phrases and nowhere else: in twain, meaning in two, and Kipling's line about east and west. Mark Twain took his name from a riverboat call meaning two fathoms — mark twain was the depth at which the water was just safe.
1nountwainTwo, in old use.
“The ship's mast was snapped in twain by the storm.”
twain is a number.
Old English twegen is the masculine form of two. The other forms were lost and twain survived alongside two.
Proto-Indo-European
*dwóh₁
Middle English
tweyne
English, today
twain
From Middle English tweyne. From Middle English tweyne, tweien, twaine, from Old English twēġen (“two”), from Proto-West Germanic twai-, from Proto-Germanic twai, from Proto-Indo-European dwóh₁. Cognate with stq twäin, 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 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.
Etymology from Wiktionary, CC BY-SA