Where does the saying hair of the dog come from?

The full saying is more like "the hair of the dog that bit you". Weirdly enough, it was originally meant to be taken literally. Placing the fur of the dog that bit you on the wound was considered to be a cure for rabies. The saying can be traced back to a Latin saying,similia similibus curentur, meaning "like cures like", which dates back to around the 4th century BC. The use of the phrase as a metaphor for drinking more alcohol to cure a hangover dates back to the fifteenth century.