Webster's English Dictionary

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countersink

1. coun.ter.sink \'kau.nt-*r-.sink\ vt 1: to make a countersink on  2: to 
   set the head of (as a screw) at or below the surface 
2. countersink n 1: a funnel-shaped enlargement at the outer end of a 
   drilled hole  2: a bit or drill for making such an enlargement