“Hair of the dog” is usually a short form of “the hair of the dog that bit you,” a familiar name for treating a hangover with another drink. The earliest reliably documented printed use usually cited is in John Heywood’s 1546 collection of English proverbs. There, the phrase appears in a request for a drink after a night of drinking. That early appearance shows the hangover sense was already established; it does not tell us exactly when or how the saying began.
The longer expression invites a literal explanation: putting hair from a dog on a wound made by that dog, supposedly to help it heal. Such a remedy is often offered as the phrase’s origin, but the evidence does not establish that it gave rise to the saying. In particular, the 1546 example is already about drinking, not treating an animal bite. The folk remedy may help explain the image, but it should not be presented as a proven origin story.
The saying’s logic is that a little of what caused an ailment might also relieve it. In the hangover version, another alcoholic drink may temporarily dull discomfort, but it does not cure the hangover and can prolong recovery. The phrase’s medical-sounding history should not be mistaken for medical advice: neither alcohol nor dog hair is a reliable treatment for a serious bite.
Over time, “hair of the dog” became a compact, often humorous way to refer to a morning-after drink, especially one chosen because it resembles what someone drank the night before. It can also be used more broadly for a small dose of the thing blamed for an unpleasant feeling. “After last night’s wedding reception, Marco ordered a Bloody Mary as hair of the dog.” Or: “She joked that another hour of sleep was her version of hair of the dog.”
The phrase endures because its vivid little image makes a familiar temptation sound like an old remedy.