The Collaborative International Dictionary of English v.0.48:

Countersink \Coun"ter*sink`\ (koun"t[~e]r*s[i^][ng]k`; 277), v.
   t. [imp. & p. p. Countersunk (-s[u^][ng]k`); p. pr. & vb.
   n. Countersinking.]
   1. To chamfer or form a depression around the top of (a hole
      in wood, metal, etc.) for the reception of the head of a
      screw or bolt below the surface, either wholly or in part;
      as, to countersink a hole for a screw.
      [1913 Webster]

   2. To cause to sink even with or below the surface; as, to
      countersink a screw or bolt into woodwork.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Countersunk \Coun"ter*sunk`\ (-s?nk`), p. p. & a. from
   Countersink.
   1. Chamfered at the top; -- said of a hole.
      [1913 Webster]

   2. Sunk into a chamfer; as, a countersunk bolt.
      [1913 Webster]

   3. Beveled on the lower side, so as to fit a chamfered
      countersink; as, a countersunk nailhead.
      [1913 Webster]