counterbore

US: (koun′tər bôr′, -bōr′; koun′tər bôr′, -bōr′)

WordReference Random House Unabridged Dictionary of American English © 2025

coun•ter•bore  (koun′tər bôr′, -bōr′; koun′tər bôr′, -bōr′), n., v., -bored, -bor•ing. 
n. 

1. Building a tool for enlarging a drilled hole for a portion of its length, as to permit sinking a screw head.
2. Building the portion of the hole so enlarged.

v.t. 

3. Building to enlarge (a drilled hole) along part of the length of the hole by drilling with a bit having a greater diameter.

• counter- + bore¹ 1605–15

coun′ter•bor′er, n.