counterbore noun. ['kaʊntǝbɔ:] L19.
[from COUNTER- 2 + BORE noun¹.]
1. A drilled hole that has a flat-bottomed enlargement at its mouth. L19.

2. A drill whose bit has a uniform smaller diameter near the tip, for drilling counterbores in one operation. L19.

[TahlilGaran] English Dictionary ▲

counterbore verb. ['kaʊntǝbɔ:; in sense 1 kaʊntǝ'bɔ:] E17.
[Sense 1 from COUNTER- 1 + BORE verb¹; sense 2 from the noun.]
1. verb intrans. Bore in the opposite direction. Now rare or obsolete. E17.

2. verb intrans. & trans. Drill a counterbore (in). L19.

[TahlilGaran] English Dictionary ▲