In motion or being stirred; set in motion or agitated.
From Old English 'stir' plus the intensive prefix 'a-', meaning to set something in motion or to agitate. This formation follows the same pattern as 'astir,' which is still used in modern English phrases like 'all astir.'
While 'astir' is its more famous sibling still used today, 'asteer' shows us that early English speakers had multiple ways to say the same thing—sometimes we keep one variant and forget the others, turning them into linguistic fossils.
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