What is "northumberland burr"

Burr \Burr\ (b[^u]r), n. [See Bur.] (Bot.)

1. A prickly seed vessel. See Bur, 1.

2. The thin edge or ridge left by a tool in cutting or shaping metal, as in turning, engraving, pressing, etc.; also, the rough neck left on a bullet in casting.

   The graver, in plowing furrows in the surface of the copper, raises corresponding ridges or burrs.
   --Tomlinson.

3. A thin flat piece of metal, formed from a sheet by punching; a small washer put on the end of a rivet before it is swaged down.

4. A broad iron ring on a tilting lance just below the gripe, to prevent the hand from slipping.

5. The lobe or lap of the ear.

6. [Probably of imitative origin.] A guttural pronounciation of the letter r, produced by trilling the extremity of the soft palate against the back part of the tongue; rotacism; -- often called the Newcastle burr, Northumberland burr, or Tweedside burr.

7. The knot at the bottom of an antler. See Bur, n.,