"prickly seed vessel of some plants," c.1300, burre, from a Scandinavian source (cf. Dan. borre, Swed. hard-borre, O.N. burst "bristle"), from PIE *bhars- (see bristle). Transferred 1610s to "rough edge on metal," which may have been the source of the sense "rough sound of the letter -r-" (see burr).