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.
      [1913 Webster]

   2. A horse or pony kept for hire.
      [1913 Webster]

   3. A carriage kept for hire; a hack; a hackney coach.
      [1913 Webster]

   4. A hired drudge; a hireling; a prostitute.
      [1913 Webster]