In a state of boiling or coming to a boil; actively bubbling from heat.
From Middle English a-boile, combining the prefix a- (in or at a state of) with boil, from Old English bīlian, related to Old Norse benja (to swell).
The prefix 'a-' in 'aboil' is an archaic way of turning verbs into adjectives describing a state—the same prefix appears in 'ablaze' and 'asleep,' which are much more common today.
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