Pimping is basically a series of questions about a certain medical condition, lab test, treatment, etc. Its basically like a nasty pop quiz in the middle of rounds, done to both test your knowledge and depending on the attending, assert his/her superiority.
The student agrees to provide certain "services" for clients of the doctor. The doctor keeps all the money and the student hopes for a few extra credit points on their eval.
In all seriousness it is when an attending asks a med student (or resident) questions (usually of the obscure variety, in a public setting, and in a somewhat demeaning fashion).
In short, it the tradition of attendings/residents publically asking challenging questions to med students or lower level residents/interns for the purpose of probing their knowledge or in general to reinforce the medical hierarchy through domination.