countersink

UK: /ˈkaʊntə^rsɪŋk/

US: (v., n. koun′tər singk′; v. also koun′tər singk′)

Inflections of 'countersink' (v): (⇒ conjugate)

countersinks

v 3rd person singular

countersinking

v pres p

countersank

v past

countersunk

v past p

WordReference Random House Unabridged Dictionary of American English © 2025

coun•ter•sink  (v., n. koun′tər singk′;v. also koun′tər singk′), v., -sank, -sunk, -sink•ing, n. 
v.t. 

1. Building to enlarge the upper part of (a cavity), esp. by chamfering, to receive the cone-shaped head of a screw, bolt, etc.
2. Building 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.

n. 

3. Building a tool for countersinking a hole.
4. Building a countersunk hole.

• counter- + sink 1810–20

Collins Concise English Dictionary © HarperCollins Publishers::

countersink / ˈkaʊntəˌsɪŋk/ verb ( -sinks, -sinking, -sank, -sunk)(transitive)

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

noun

1. Also called: countersink bit a tool for countersinking
2. a countersunk depression or hole