“Hair of the dog” is a shortened form of the older expression hair of the dog that bit you. Today, it means drinking a small amount of alcohol the next morning to reduce hangover symptoms.
The image behind the phrase is much older than the modern hangover meaning. In folk medicine, people once believed that if a dog bit you, a hair from that same dog could help cure the wound. The idea was that the source of the harm could also be the source of the remedy.
That old logic later shifted into drinking culture. If alcohol caused your headache or nausea, then a little more alcohol might make you feel better, at least for a short time. That is why the idiom now refers to a morning drink after a night of heavy drinking.
- Modern meaning: a drink taken to ease a hangover
- Tone: usually informal, often humorous
- Example: He said a beer at brunch was hair of the dog after the party.
People usually use the phrase jokingly, not as medical advice. It is mainly a colorful idiom about trying to fix a problem with a little of the thing that caused it.