In two; into two pieces or parts (archaic or dialectal).
From Old English prefix 'a-' (in, on, or into a state) + 'two.' This was a productive construction in Middle English where 'a-' could be attached to numbers or conditions to describe a state or position. Related to similar formations like 'atone' (at one) and 'aloof' (on the luff).
English used to have all these 'a-number' expressions—'atone,' 'atwo'—like they were super common ways to describe states, but most got lost. You can still hear echoes in phrases like 'all in one' or 'one by one,' showing how differently people used to play with prepositions and numbers.
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