To listen carefully, or to listen to someone with attention; often used in the phrase 'harken back' meaning to recall or pay attention to something from the past.
From Old English 'heorcnian' meaning 'to listen,' related to 'hear.' The word is archaic in modern English but appears frequently in the phrase 'harken back,' which technically should be 'hark back' (harken is already a verb meaning listen).
English speakers have been using 'harken back' for 200+ years even though it's technically a double verb (like saying 'listen listen back'), but it's so useful that language experts just accepted it as correct—which shows how living languages ignore grammar rules when they're practical.
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