burr

(historical) A metal ring at the top of the hand-rest on a spear.

1724, John Guillim, A Display of Heraldry:

burr or ring of iron behind the hand

1819, Abraham Rees, The Cyclopaedia:

The front of it was defended by an iron-plate, called a vam-plat, that is, an avant-plate, and behind it was a broad iron ring, called a burr.

2003, Thomas Howard Crofts, Fifteenth-century Malory, page 290:

We are made to witness a cathartic shuffling-off of mortalities and of hatreds: Mordred's pulling himself up to the 'burr' of Arthur's spear is Malory's own detail and one of the most memorable in the book.

2012, Howard Pyle, The Story of the Grail and the Passing of Arthur:

And when his body was against the burr of the spear, he took his sword in both his hands and he swung the sword above his head, and he smote King Arthur with the edge of the sword upon the helmet.

2015, James B. Tschen-Emmons, Artifacts from Medieval Europe, page 280:

Many saddles, especially those for use on warhorses, had high burr plates and cantles. this was especially important when knights began using stirrups and the couched lance.