hairpin

The noun refers to a specific functional object, typically associated with grooming and hairstyling. It is a concrete noun used in everyday contexts, though it can occasionally appear in historical or literary descriptions of attire.

As an adjective, the term is used almost exclusively in the context of geography, civil engineering, and racing. It describes a bend so sharp that it resembles the U-shape of the physical pin. This usage is highly specific to roads, tracks, or paths and is rarely applied to other types of curves.