The idea of having a ‘hair of the dog (that bit you)’ was once entirely literal. In the Middle Ages, anyone bitten by a stray dog would run after the offending animal in an attempt to pluck out one of its hairs.
A poultice with that hair was believed to greatly ease the post-drinking blues (what in German they call a Katzenjammer, in which the drinker’s moans are compared to the wailing of a very miserable cat).
How many people managed to get bitten again when trying to approach the aforesaid dog to acquire the hair to achieve this completely useless remedy isn’t known.
John Heywood, in his early text, A dialogue conteinyng the nomber in effect of all the prouerbes in the Englishe tongue, 1546, uses a clear drinking reference:
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.
Robert James alludes 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.
James is rather skeptical about the treatment, preferring a more mainstream method- the application of the ashes of river crabs.
Via: BBC Culture