i have no idea, but this is what wiki says,
Hair of the dog" is a colloquial expression in the English language predominantly used to refer to alcohol that is consumed with the aim of lessening the effects of a hangover. The expression originally referred to a method of treatment of a rabid dog bite by placing hair from the dog in the bite wound.[1] The use of the phrase as a metaphor for a hangover treatment dates back to the time of William Shakespeare[citation needed]. Ebenezer Cobham Brewer writes in the Dictionary of Phrase and Fable (1898): "In Scotland it is a popular belief that a few hairs of the dog that bit you applied to the wound will prevent evil consequences. Applied to drinks, it means, if overnight you have indulged too freely, take a glass of the same wine within 24 hours to soothe the nerves. 'If this dog do you bite, soon as out of your bed, take a hair of the tail the next day.'" He also cites two apocryphal poems containing the phrase, one of which is attributed to Aristophanes. It is possible that the phrase was used to justify an existing practice, and the idea of Latin: similia similibus curantur ("like cures like") dates back at least to the time of Hippocrates.