Between; in the space separating two things (an archaic or poetic form of 'betwixt').
From Middle English 'atwixt' or 'atwix,' a variant of 'betwixt.' The 'a-' prefix comes from Old English meaning 'in' or 'on,' combined with 'twixt' (from 'betwixt'), which itself derives from Old English 'be-' (by) + 'twi-' (two). The word literally meant 'by the two' or 'between the two.'
This is essentially Shakespeare-era fancy talk for 'between,' and you'll find it in old poetry where it sounds way more dramatic than saying 'betwixt.' The 'a-' prefix trick—slapping it on the front of words to make them mean 'in a state of'—was super common in Middle English and explains why so many old words start with 'a' like 'abed,' 'ablaze,' and 'afoot.'
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