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general::
Whatever made you ill used as a remedy, especially alcohol as a hangover cure. For example, A little hair of the dog will cure that hangover in no time. This expression, already a proverb in John Heywood's 1546 compendium, is based on the ancient folk treatment for dogbite of putting a burnt hair of the dog on the wound. It is often shortened, as in the example.
American Heritage Idioms