'atwain' originates from Old English elements: the prefix 'a-' (from Old English 'on' or 'an', meaning 'in' or 'on') combined with 'twain' (Old English 'twēgen'/'twa', meaning 'two').

'atwain' appears in Middle English as phrases like 'at twain' or fused forms 'atwain', derived from Old English compounds and expressions meaning 'in two'. Over time the fused form 'atwain' was used in poetic and dialectal contexts and eventually became archaic.

Initially it meant 'in two' or 'into two parts', and that core sense has been preserved; what changed is usage frequency—it's now archaic/literary rather than part of everyday speech.