bristling

Bristling is the act or condition in which hairs or spines on the skin stand upright, creating a rough, prickly surface. It can also refer to a surface that is covered with many short, stiff projections, such as the bristles on a brush. As a verb, bristle means to react with anger or indignation, or to be full of sharp points or projections.

In biology, the phenomenon is called piloerection. Tiny muscles at the base of hair follicles, the arrector pili, contract and lift the hairs. Triggers include cold, fear, aggression, touch, and certain environmental cues. Piloerection provides insulation by trapping air in fur and can exaggerate an animal's size as a defensive display.

Figuratively, people and things can bristle with something, meaning they are filled with or covered in it: for example, an argument bristling with objections, or a surface bristling with needles. The phrase is often used to convey tension or abundance of edges rather than literal hairs.

In botany and horticulture, some plants possess bristling hairs or trichomes that deter herbivory. In everyday objects, bristles are the stiff hairs of brushes or combs; bristling describes those projections as a characteristic of the object.