Why is shiner slang for a black eye?

The term "shiner" for a black eye is thought to have originated from the shiny appearance of a bruised eye. The discoloration caused by a black eye often reflects light in a way that makes it appear shiny or glossy.

There's no definitive historical record of the term's origin, but this explanation seems plausible given the visual characteristic of a black eye.

However, some sources suggest that the term "shiner" could also be connected to the shininess of the eyelid when it's swollen due to bruising.

Regardless of the exact origin, "shiner" is a widely recognized and commonly used slang term for a black eye.