The Collaborative International Dictionary of English v.0.48:

Calkin \Calk"in\, n.
   A calk on a shoe. See Calk, n., 1.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Calk \Calk\ (k[add]k), n. [Cf. AS. calc shoe, hoof, L. calx,
   calcis, heel, calcar, spur.]
   1. A sharp-pointed piece of iron or steel projecting downward
      on the shoe of a horse or an ox, to prevent the animal
      from slipping; -- called also calker, calkin.
      [1913 Webster]

   2. An instrument with sharp points, worn on the sole of a
      shoe or boot, to prevent slipping.
      [1913 Webster]

   3. same as caulk[2], n..
      [PJC]

WordNet (r) 3.0 (2006):

calkin
    n 1: a metal cleat on the bottom front of a horseshoe to prevent
         slipping [syn: calk, calkin]