Positioned against or upon a rock; in contact with or resting on rock.
From Old English prefix 'a-' (meaning 'on' or 'at') combined with 'rock'. Like 'aroast,' this represents the productive Middle English pattern of using 'a-' to describe positional states.
Medieval English had this beautiful way of building position-words with 'a-': 'arock,' 'aflame,' 'aground'—it was grammatically productive until we invented prepositions like 'on' and 'at' to do the same job, making these forms gradually obsolete.
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