Definition of Hair of the dog

Hair of the dog: An alcoholic chaser reputed to relieve a hangover, usually needed the morning after imbibing too much alcohol. There is no scientific evidence as to the efficacy of this time-honored treatment for a hangover.

The saying originated in the belief that a cure for hydrophobia (rabies) or any disease contracted from a dog bite consisted of taking a hair of the dog that bit you and placing it in the wound.