The present participle of 'hackney'; the act of making something ordinary or overused, or renting out horses for hire.
From 'hackney' (verb: to use excessively or make common). The word preserves an older meaning where 'hackney' meant to use something repeatedly until it loses value or freshness.
When writers talk about 'hackneying' an idea, they mean overusing it to death—it's like borrowing the same horse so many times that everyone recognizes it and it loses its appeal.
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