A plum bob is an alchemical poison that is absorbed through the skin. Its name is derived from the taste of plum and often nutmeg which follows its ingestion.
A plum bob is a toxic substance discussed in Alchemy at The University. It is very dangerous to handle, as its compounds can be absorbed through the pores. The results of consuming this poison vary from person to person, but its most infamous effect is the negation of the victim's "social filters" - making it difficult or nearly impossible to distinguish the difference between right and wrong. When strategically administered, poisoning by plum bob can result in the victim taking actions that are at worst dangerous or destructive, or at best severely embarrassing (at least after the effects have worn off).
The effects typically last around eight hours, but as a plum bob is a lipid-soluble poison, it can remain in the system and cause relapses long after initial exposure. Many of these residual effects also vary, though they are often provoked by emotional distress.
In The Chronicle
Before Kvothe's second admissions interview, he is made the victim of a plum bob by a beautiful Modegan woman while in the common room of Anker's. Ultimately, it is discovered the woman did this at the behest of Ambrose Jakis who had managed to procure the potion from an associate of Devi. It is later revealed she personally made the dose.
At first, Kvothe is unaware what is happening to him and confused, goes to Simmon for advice. Sim's experience with Alchemy reveals the cause for Kvothe's outbursts, thus forcing him to switch admission tiles with Fela.
Eventually, as the effects of the poison subside, Kvothe is allowed to return to his room at Anker's. He subsequently locks himself inside for several days as he relives the trauma of his families murder as well as uncontrollably mourning their loss. Eventually, Auri finds him in his room, and tries to console him, where an emotional Kvothe laments his mother's death most specifically.
Throughout the rest of the chronicle, Kvothe experiences relapses often prefaced by the taste of plum and nutmeg. During these regressions, he is subject to emotional outbursts as well as considerations his character probably would not have innately made.