The Collaborative International Dictionary of English v.0.48:

Holdback \Hold"back`\, n.
   1. Check; hindrance; restraint; obstacle.
      [1913 Webster]

            The only holdback is the affection . . . that we
            bear to our wealth.                   --Hammond.
      [1913 Webster]

   2. The projection or loop on the thill of a vehicle. to which
      a strap of the harness is attached, to hold back a
      carriage when going down hill, or in backing; also, the
      strap or part of the harness so used.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "holdback":
   arrest, arrestation, arrestment, bearing rein, bit, blockage,
   blocking, brake, chain, check, checkrein, chock, clog, clogging,
   closing up, closure, constriction, countercheck, cramp, curb,
   curb bit, damper, delay, detainment, detention, doorstop, drag,
   drag sail, drift anchor, drift sail, drogue, fetter, fixation,
   foot-dragging, hampering, hindering, hindrance, holdup, impediment,
   inhibition, interference, interruption, let, martingale,
   negativism, nuisance value, obstruction, obstructionism, occlusion,
   opposition, pelham, remora, repression, resistance, restraint,
   restriction, retardation, retardment, scotch, sea anchor, setback,
   shackle, snaffle, spoke, squeeze, stay, stop, stranglehold,
   stricture, suppression, trammel