In medieval times, people believed that if you were bitten by a rabid dog, you'd be cured if you plucked some hair from said nasty canine and applied it to your wound. Around this same time, people also began to profess that if you suffered from a hangover, you could cure it by drinking a little more hooch – i.e., partaking of "the hair of the dog that bit me." This phrase was actually recorded as far back as 1546 by John Heywood in "A dialogue conteinyng the number in effect of all the prouerbes in the Englishe tongue" [source: Martin].
Unfortunately, this longstanding belief is inaccurate. It's true that if you sip a bit of alcohol the day after you over-imbibe, you won't have a hangover -- not right away, at least. Hangovers start knocking on the door when your blood-alcohol level begins to drop; the pain is the worst when there's no alcohol left in your system. So if you pour a little more down your gullet, you'll prop up that level and forestall the hangover. But at some point you'll have to face the music and stop drinking. Your blood-alcohol level will then drop, and the hangover will hit. Rather than reaching for the bottle, you'll be better served guzzling some water or a sports drink, the latter of which can help you replace lost electrolytes as well as rehydrate you [source: Hudepohl].