"The hair of the dog"
definition: A small measure of drink, intended to cure a hangover.
origin: The fuller version of this phrase, i.e. 'the hair of the dog that bit me', gives a clue to the source of the name of this supposed hangover cure. That derivation is from the mediaeval belief that, when someone was bitten by a rabid dog, a cure could be made by applying the same dog's hair to the infected wound.