Oil of Scallatine
Though the precise nature of oil of scallatine is not known, it is presumably rare, and very smelly. In the city of Ankh-Morpork, the total quantity of scallatine available in the city at the very most measures up to an ounce, and the largest amount customers are usually allowed is about the size of a pinhead. Which is a good thing, as any more and it may be considered a chemical weapon of mass destruction.
Handling the material is extremely hazardous. It never washes off, and if accidentally released, will effectively overload the sinuses of those present. To prevent leakage of the foul contents, the oil is stored in a container submersed in water.
In The Truth, William de Worde planned to make use of an ounce of the liquid to construct a smell bomb to ward off a certain werewolf who works for the City Watch. Even a pinhead of the liquid was enough to incapacitate for an entire day. Angua von Uberwald initially described it as the equivalent of going around a corner and walking into Foul Ole Ron and his Smell, though she later revised this statement.
A practical equivalent might be allyl isocyanide, which has been suggested as a non-lethal weapon due to its devastatingly foul odor.