The Dragon's Whisker is, as its name suggests, a whisker taken from the face of a Chinese Dragon. It was harvested almost four thousand years ago by a famous Chinese warlord who was suffering a crisis of confidence due to his premature baldness.
The whisker can be used to make stock for rice porridge. Though it only affects men, whoever eats the whisker-stock porridge experiences dramatic hair growth.
Bald men will regain a life-long supply of luscious, healthy hair.
But men who have hair already will find their hair growth accelerating to impossible levels, causing their hair to visibly grow to massive lengths with such ferocity that, after a week of unrestrained growth, the victim would go bald. It's implied that even a fresh dose of the whisker will be unable to restore the hair of such an unfortunate.
Fortunately, by tying their hair up with the dragon's whisker, such an unfortunate can suppress the hair-growth effect.
Ranma Saotome unwittingly ate Dragon's Whisker Porridge during his travels in China, after receiving his Jusenkyo curse. Fortunately, the Dragon's Whisker Salesman was willing to give Ranma the Dragon's Whisker, which Ranma subsequently used to restrain his cursed hair.
Eventually, Ranma's past caught up to him when a group of four thugs, the Niku-men, pursued him from China in hopes of retrieving the whisker. When Happosai and Genma Saotome realised the whisker's magic, they too got involved and subsequently the whisker was stolen from Ranma.
Fortunately, the Dragon's Whisker has a shelf life of 4000 years, and it reached its expiry date during the conflict. This made the whisker useless, and caused its magic to depart from Ranma, leaving him free of that particular curse.