Present participle of 'hark': listening attentively, or the act of hearing something.
From 'hark' (Old English heorcnian) plus the gerund/participle suffix '-ing.' This creates the continuous form and allows 'harking' to function as a noun, adjective, or verb form.
'Harking back' is probably the only phrase where modern English speakers instinctively know what 'hark' means—it's a mysterious linguistic island where an archaic word survives because of one useful phrase.
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