"stiff, coarse hair of certain animals," especially those set along the backs of hogs, Old English byrst "bristle," with metathesis of -r-, from Proto-Germanic *bursti- (source also of Middle Dutch borstel, German borste, Danish börste), from PIE *bhrsti- from root *bhars- "point, bristle" (source also of Sanskrit bhrstih "point, spike"). With -el, diminutive suffix. Extended to similar appendages on some plants and insects.
c. 1200 (implied in past-participle adjective bristled) "set or covered with bristles," from bristle (n.). Of hair, "to stand or become stiff and upright," late 15c. Extended meaning "become angry or excited" is 1540s, from the way animals show fight. Related: Bristling.