A Salty Dog is a cocktail of gin and grapefruit juice, served in a highball glass with a salted rim. The salt is the only difference between a salty dog and a greyhound. Historically a gin drink, it is believed to date back to the 1920s.
The drink is 2 fluid ounces of gin mixed with fresh grapefruit juice, shaken, and poured into a glass whose rim has been salted.