In a wide state or manner; spread out widely or broadly.
From 'a-' plus 'wide.' This represents an archaic or poetic way of expressing wideness as a continuous state of being, following Middle English adverbial patterns.
You can see how creative English speakers were in the medieval period—by prefixing 'a-' to adjectives and nouns, they could instantly create adverbs that painted vivid pictures of position and state!
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