The Collaborative International Dictionary of English v.0.48:

Hackney \Hack"ney\ (-n[y^]), n.; pl. Hackneys (-n[i^]z). [OE.
   hakeney, hakenay; cf. F. haguen['e]e a pacing horse, an
   ambling nag, OF. also haquen['e]e, Sp. hacanea, OSp. facanea,
   D. hakkenei, also OF. haque horse, Sp. haca, OSp. faca; perh.
   akin to E. hack to cut, and nag, and orig. meaning, a jolting
   horse. Cf. Hack a horse, Nag.]
   1. A horse for riding or driving; a nag; a pony. --Chaucer.
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   2. A horse or pony kept for hire.
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   3. A carriage kept for hire; a hack; a hackney coach.
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   4. A hired drudge; a hireling; a prostitute.
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The Collaborative International Dictionary of English v.0.48:

Hackney \Hack"ney\, a.
   Let out for hire; devoted to common use; hence, much used;
   trite; mean; as, hackney coaches; hackney authors. "Hackney
   tongue." --Roscommon.
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The Collaborative International Dictionary of English v.0.48:

Hackney \Hack"ney\, v. t. [imp. & p. p. Hackneyed (-n[i^]d);
   p. pr. & vb. n. Hackneying.]
   1. To devote to common or frequent use, as a horse or
      carriage; to wear out in common service; to make trite or
      commonplace; as, a hackneyed metaphor or quotation.
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            Had I so lavish of my presence been,
            So common-hackneyed in the eyes of men. --Shak.
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   2. To carry in a hackney coach. --Cowper.
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WordNet (r) 3.0 (2006):

hackney
    n 1: a carriage for hire [syn: hackney, hackney carriage,
         hackney coach]
    2: a compact breed of harness horse