Hair of the Dog

The full, older version of this phrase is "a hair of the dog that bit you," and it originally had nothing to do with hangovers. Going back at least to Roman times, there was a genuine folk medical belief, described by Pliny the Elder among others, that a dog bite (even from a rabid dog) could be treated by placing a hair from that same dog onto the wound, on the logic that like cures like, the same idea behind homeopathy. By the time English writer John Heywood recorded the phrase in his 1546 collection of proverbs, it already carried the modern figurative sense: curing the effects of drink with a little more of the same drink.