At full speed; with all one's force or strength; with great violence or intensity (archaic).
From Middle English 'amain,' from Old English 'on mægne' (at full strength/might), literally 'on might.' The 'a-' is a preposition and 'main' comes from Germanic roots for strength.
Shakespeare and Marlowe loved 'amain' because it packed so much action into one word—instead of saying 'with full strength,' they'd just say 'amain' and suddenly their actors were tearing across the stage at top speed, and audiences felt the urgency.
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