In a cool manner; coolly, or referring to a state of being cool; an archaic or dialectal form.
From a- (prefix used to create adverbs) + cool, following an older English pattern where the 'a-' prefix created verbal forms, as in 'ablaze' or 'asleep.' Rarely used in modern English.
The 'a-' prefix in words like acool is a ghost from Old English—it's the same prefix that survives in words like 'afraid' and 'alive,' showing that English once formed adverbs very differently than today.
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