the.com/bristle
What hair does when it gets defensive — on a pig, a brush, or a person.
means To stiffen short stiff hairs in alarm, or to react with sudden visible indignation.
from From Old English byrst, a stiff hair, plus a frequentative twist — the word literally is the hairs standing up that it describes.
Pig rootsOriginal bristles were hog hair, prized for brushes.
Toothbrush DNAFirst nylon bristles replaced boar hair in 1938.
Idiom shiftFrom hairs erecting to humans taking offense.