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]