Hair of the Dog

Hair of the Dog (that bit you): an alcoholic drink that is supposed to cure a hangover

Happy Saturday! Hopefully you didn't need any 'hair of the dog' this morning...

This phrase was originally in reference to the treatment of a rabid dog bite. You were supposed to put the dog's hair on your wound and you would be cured. I'm not sure how effective that treatment could possibly be though, since there is currently no known cure for rabies. The only way modern medicine can treat the disease is to administer a vaccine within two days of being bitten. Untreated, the disease can lead to coma and death.
The modern use of 'hair of the dog' dates back to Shakespeare himself who used the phrase in Twelfth Night. There is some evidence that this is an effective treatment because low doses of ethanol can alleviate the first stages of alcohol withdrawal. However, a hangover is not exactly the same as withdrawal and introducing more alcohol into your system is a questionable move when you are dehydrated and feeling like sh*t anyway.
I'll stick to ibuprofen and large quantities of water...and breakfast from Micky-D's