An archaic form meaning in a state of hunting, or engaged in the hunt; to be actively searching for something.
From the 'a-' prefix (in a state of) plus 'hunt,' following the Middle English pattern of intensifying adjectives and verbs with this prefix.
Medieval texts describe knights as 'ahunt' to give a sense of them being fully engaged in the action—it's like how we say someone is 'on the hunt' today, but more poetically intense.
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