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(Non-english questioner) To countersink means to drill a sloped hole so that screw heads can sink into the material to be hidden.

In German for example the term for this action is "senken", which would translate to only "sink" (not "countersink", which would literally translate to German "gegensenken" - which doesn't make any sense in German).

I know the followng meanings of "counter":

  • a "mechanism" to increase numbers
  • something like "opposite" or "reverse" (like in "counterattack")
  • the furniture in a shop or bar where the salesperson works

None of them makes sense for screws. What exactly does "counter" mean here?

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  • Etymology. From counter- (“against”) +‎ bore (“hole”). See also 'counterbore' (a cylindrical variation). – Michael Harvey Oct 28 '20 at 19:58
  • @MichaelHarvey But do I really sink a screw "against" the hole? I only sink the screw, don't I? – Foo Bar Oct 28 '20 at 20:02
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    Since the pocket for the screwhead is secondary to the hole the screw drives into, that may make the counter part. When you counter an offer, you make a secondary offer. – Yosef Baskin Oct 28 '20 at 20:07
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    Makes sense to me. You "counter" the conical shape of the bottom of a flat-head screw. – Hot Licks Oct 28 '20 at 20:08
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    "Countersink" can also refer to a cylindrical depression to accommodate a cylindrical screw head (such as a socket-head cap screw) so the top sits below flush with the surface. – Spehro Pefhany Oct 29 '20 at 4:51
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The conjecture on this in the OED says

Etymology: counter- prefix, apparently in a sense akin to 8 b, the hole being the counterpart of that which is to be sunk in it.

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  • Thanks. So the screw is sunk into the countersink. People always say "countersinking the screw", which would be wrong I believe. They countersink (the hole) first and simply sink the screw into it afterwards. – Foo Bar Oct 28 '20 at 20:23
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    Screws with conical heads designed for these holes are called "countersunk screws" in the fastener trade. – Michael Harvey Oct 28 '20 at 20:28
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My grandfather was a blacksmith (now long dead). My understanding is that a countersink (also called a bob-punch), is used to form a depression in the surface of plates to be joined together when rivetting. Without this, the head of the rivet projects entirely and is subject to damage. If the head is broken off, the rivet will sink into the plate. The indentation counters any tendency to sink. I suspect the term also applies to indentations for horseshoe nails.

Unfortunately my grandfather and my father are deceased and I can't find anything online to support what I'm saying.

I'll leave this here for now and see if I can find anything further.

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    The term also applies to wood screws with conical heads that go in flush with the wood surface. I don't dispute the blacksmithing meaning you present, but be aware that it's not restricted to blacksmithing. IDK which meaning came first. – Peter Cordes Oct 29 '20 at 4:30
  • And yet, it seems to be the same meaning. If you used a screw with a round head, it would protrude out of the wood and risk being sheared off, in which case, the body of the screw would sink further into the wood. – João Mendes Oct 29 '20 at 9:05
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Google's Ngram viewer returns results for 'countersink' that indicate that it was a new word in the early 19thC. It does not appear in the corpus at all before 1820 and did not take off until about 1860. This indicates that "countersinking" was a process associated with industrialisation.

The Lexico entry for counter gives an uncommon definition which seems to be relevant.

The back part of a shoe or boot, enclosing the heel.

For which it gives the origin as

Mid 19th century abbreviation of counterfort ‘buttress’, from French contrefort.

The mid 19thC origin of the shoemaking term matches quite well with the appearance of "countersink" in the literature and "countersinking" a hole so that the head of a screw or other fastener fits below the surface certainly improves the strength of the join and could be said to "buttress" the fastener. I suspect that "countersink" also derives, perhaps in part, from counterfort.

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