[koun-ter-bawr, -bohr; koun-ter-bawr, -bohr]
  1. a tool for enlarging a drilled hole for a portion of its length, as to permit sinking a screw head.
  2. the portion of the hole so enlarged.
verb (used with object), coun·ter·bored, coun·ter·bor·ing.
  1. to enlarge (a drilled hole) along part of the length of the hole by drilling with a bit having a greater diameter.

Origin of counterbore

First recorded in 1605–15; counter- + bore2
Related formscoun·ter·bor·er, noun Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Examples from the Web for counterbore

Historical Examples of counterbore