[v., n. koun-ter-singk; v. also koun-ter-singk]
verb (used with object), countersank, countersunk, countersinking.
to enlarge the upper part of (a cavity), especially by chamfering, to receive the cone-shaped head of a screw, bolt, etc.
to cause (the head of a screw, bolt, etc.) to sink into a prepared depression so as to be flush with or below the surface.
a tool for countersinking a hole.
a countersunk hole.

1810–20; counter- + sink Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
countersink (ˈkaʊntəˌsɪŋk)
vb , -sinks, -sinking, -sank, -sunk
1.  to enlarge the upper part of (a hole) in timber, metal, etc, so that the head of a bolt or screw can be sunk below the surface
2.  to drive (a screw) or sink (a bolt) into such an enlarged hole
3.  Also called: countersink bit a tool for countersinking
4.  a countersunk depression or hole

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Example sentences
Too slow a speed will cause the countersink to chatter.
May countersink nails or screws below surface of wall prior to applying sealing
  compound using hammer or screwdriver.
Countersink nails or screws below surfaces of walls before applying sealing
  compounds, using hammers or screwdrivers.
Countersink bolts and nuts flush with surfaces, unless otherwise indicated.
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