I actually had to look this one up, as I the message behind the ‘hair of the dog that bit you‘ expression is pretty obvious, but the actual origin is likely to be one of those “huh, what-do-you-know” type stories.
A little bit of digging revealed the origin. the expression dates all the way back to the Middle Ages, when people treated a dog bite with the ashes of the canine culprit’s hair (those crazy medival doctors).
But they can’t take credit for the genius thinking behind the treatment, that medical logic came from the Romans; who believed that the cure of any ailment, including a hangover, could be found in its cause.
It’s a principle applied in modem medicine with the use of vaccines for immunization.
Essentially, “The hair of the dog” treatment for hangovers advises that to feel better, you should take another drink of the same thing that made you feel sick.
…Personally, I’ll stick to a hangover cure that I know actually works, and keep the ‘hair of the dog’ myth for those days when I just want to stay drunk for longer 😀