At work; engaged in work or labor; in operation or functioning.
From 'a-' (Old English prefix) + 'work,' an archaic construction equivalent to modern 'at work.' Similar to how 'afoot' means 'at foot' or 'in progress.'
The 'a-' prefix in words like 'awork,' 'afoot,' and 'aflame' shows how English once used spatial prepositions differently—'a-' literally meant 'at' or 'in' before words became more compact.
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