Why is the ninth month called September?
"prickly seed vessel of some plants," c.1300, burre, from a Scandinavian source (cf. Danish borre, Swedish hard-borre, Old Norse burst "bristle"), from PIE *bhars- (see bristle (n.)). Transferred 1610s to "rough edge on metal," which might be the source of the sense "rough sound of the letter -r-" (see burr).
"rough sound of the letter -r-" (especially that common in Northumberland), 1760, later extended to "northern accented speech" in general. Possibly the sound of the word is imitative of the speech peculiarity itself, or it was adapted from one of the senses of bur (q.v.), perhaps from the phrase to have a bur in (one's) throat (late 14c.), which was a figure of speech for "feel a choking sensation, huskiness." OED says the Scottish -r- is a lingual trill, not a true burr.
bur or burr (bûr)
A rotary cutting instrument used in dentistry for excavating decay, shaping cavity forms, and reducing tooth structure.
A drilling tool for enlarging a trephine hole in the cranium.
Variant of bur.