In the process of being built; under construction.
Archaic or dialectal form using the prefix 'a-' (expressing a state or condition) combined with 'building,' the present participle of 'build' from Old English 'byldan.'
Shakespeare and other Renaissance writers loved the 'a-' prefix to show actions in progress—it created a vivid, almost cinematic sense of ongoing action that modern English has largely replaced with '-ing' forms.
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