This idiom refers to taking a small measure of the drink that gave you the hangover. It’s also come to mean, more loosely, taking a bit of the cause as the cure in a given situation.
Where did it come from?
It was a commonly held belief among ancient people, such as the Romans and Celts, that like cured like. A dog bite was believed to heal more quickly and efficiently if the same dog’s hair was applied to the wound. This also applied if the dog that bit was rabid. This was the method prescribed in medical journals. By medieval times, the prescription changed slightly to use hair of the dog that had been burned. Frankly, I can’t imagine approaching a rabid dog to get a bit of fur. I wonder how many ended up bitten in attempting the cure.
Robert James alluded to the method in A Treatise on Canine Madness, 1760:
“The hair of the dog that gave the wound is advised as an application to the part injured.”
This same mentality was applied to alcohol poisoning, or hangover. You simply gulped down a small amount of the liquor that made you drunk to take the affects away.
The first known literary use of the term “hair of the dog” appeared in 1546 in John Heywood’s A dialogue conteinyng the nomber in effect of all the prouerbes in the Englishe tongue:
“I pray thee let me and my fellow have
A hair of the dog that bit us last night –
And bitten were we both to the brain aright.
We saw each other drunk in the good ale glass.”
Just as using the hair of a rabid dog to treat a bite, I think applying this method to a hangover is likely just as unsuccessful.