A hairpin is a thin piece of wire folded into a U, used to hold long hair in place — you slide it in so the two arms press the hair together. People with long hair use them to pin a bun, a twist, or a piece that keeps falling down. Some hairpins are plain metal; some have a little decoration on the bend. Though actually, in everyday talk hairpin also names the shape: a hairpin turn (or hairpin bend) is a road curve so sharp it doubles back like that U. Drivers and cyclists use that a lot. The hair tool is ordinary, slightly old-fashioned next to clips and bands, but still common. The road sense is informal and vivid.