A hair of the dog is an alcoholic drink that someone has to try to make them feel better after drinking too much alcohol the day before. I need a drink, chum. A large hair of the dog. Note: You can also talk about a hair of the dog that bit you. Now he was feeling worse and wondering if a hair of the dog that bit him might not set him up for the day. Note: There was an old belief that if a person was bitten by a mad dog, they could be cured by putting some hairs from the dog's tail on the wound.