| Definition -
A rough sounding of the letter r.
1. According to the OED, it is specifically "the rough uvular trill … characteristic of the county of Northumberland, and found elsewhere as an individual peculiarity. (Writers ignorant of phonology often confuse the Northumberland burr with the entirely different Scotch r, which is a lingual trill…)"
Etymology - The term's derivation in this sense is unknown. However, some claim that it was coined in imitation of the speech style itself; others claim that it derives from the phrase "to have a bur in your throat."
Oxford English Dictionary - Its first citation is from 1760:
"An Aunt just come from the North, with the true NewCastle bur in her throat."
(Foote Minor (1781) Introd. 9)