An archaic or dialectal form of 'between,' indicating a position in the middle of two things.
From Old English 'atweona' or 'betwēonum,' where 'at-' combines with 'tween' (two). The 'at-' prefix was used for spatial relationships in older English.
English used to love the 'a-' prefix for positioning—'atwain,' 'atween,' 'atveen'—these variant spellings show how language was more flexible before standardization.
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