To make or become cool; an archaic or dialectal form of 'cool' used as a verb.
From Old English 'colian' (to cool, to become cold). This is an older verb form; modern English simplified 'cool' to be both noun, adjective, and verb, making 'coolen' obsolete.
Old English and Middle English had verb forms like 'coolen' that have disappeared from modern English—we just use 'cool' for everything now, showing how language simplifies verb conjugations over centuries.
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