The present participle of 'hethen,' meaning the process of heating or becoming hot.
From hethen + -ing (English gerund/present participle suffix). This is an archaic form that would have been common in Middle English but fell out of use as language simplified.
Back in older English, people loved making nouns from verbs by adding '-ing'—'hething' was their word for the whole process of heating up, just like we say 'running' or 'jumping' today.
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