Why is the ninth month called September?
Old English byrst "bristle," with metathesis of -r-, from Proto-Germanic *bursti- (cf. Middle Dutch borstel, German borste), from PIE *bhrsti- from root *bhars- "point, bristle" (cf. Sanskrit bhrstih "point, spike"). With -el, diminutive suffix.
c.1200 (implied in past participle adjective bristled) "set or covered with bristles," from bristle (n.). Meaning "become angry or excited" is 1540s, from the way animals show fight. Related: Bristling.