Twain comes from Old English twegen ('two', masculine nominative and accusative), from Proto-Germanic *twai, from Proto-Indo-European *dwo- ('two'); the masculine form survived alongside the now-standard two. The historical chain includes Old English twegen, Proto-Germanic *twai, and Proto-Indo-European *dwoh. The quoted older sense, "two," supplies the immediate semantic anchor for twain. The formation is best read as borrowed: the word twain carries its older form-history into a compact English lexical shape. In present English it is used as a noun, "two; a pair (archaic)". Related English forms include two, twin, twice, and between. The result is a noun and adj whose etymology preserves both the older form and the later English specialization.