What is "plug centerbit"

Plug \Plug\, n. [Akin to D. plug, G. pflock, Dan. pl["o]k, plug, Sw. plugg; cf. W. ploc.]

1. Any piece of wood, metal, or other substance used to stop or fill a hole; a stopple.

2. A flat oblong cake of pressed tobacco. [U. S.]

3. A high, tapering silk hat. [Slang, U.S.]

4. A worthless horse. [Slang, U.S.]

5. (Building) A block of wood let into a wall, to afford a hold for nails.

   Breech plug (Gun.), in breech-loading guns, the metal plug or cylinder which closes the aperture in the breech, through which the gun is loaded.

   Fire plug, a street hydrant to which hose may be attached.

   Hawse plug (Naut.), a plug to stop a hawse hole.

   Plug and feather. (Stone Working) See Feather, n., 7.

   Plug centerbit, a centerbit ending in a small cylinder instead of a point, so as to follow and enlarge a hole previously made, or to form a counterbore around it.

   Plug rod (Steam Eng.), a rod attached to the beam for working the valves, as in the Cornish engine.

   Plug valve (Mech.), a tapering valve, which turns in a case like the plug of a faucet.