"Woodshedding", or shedding, is a term commonly used to describe the act of practicing some endeavor, usually in private, to improve one's proficiency in performing it.
It is typically used by musicians to mean rehearsing a difficult passage repeatedly, until it can be performed flawlessly.
According to Paul Klemperer, a Texas-based jazz educator, woodshedding is more than just practicing— it is "the place where you work out the techniques that form the foundation of your improvisational ability".
[1] In legal parlance, "woodshedding" refers to the instruction given to a witness to make him respond in one party's favor.
It is also called "horse shedding" and involves practice questions and answers or even a mock trial.