Racked means to be put on a rack. It is usually found in two
contexts.
1. In pool, the balls are "racked" when they are put into a
triangular case or rack, and thus arranged on the pool table.
2. The implement of torture called "the rack" was a sort of bed
where people's feet were attached to the foot, their hands tied
together above their heads and pulled toward the head, thus
stretching them. This caused extreme pain, and could pull the
person's arms out of his or her sockets. A person tortured in this
way was said to be "racked", and this gives rise to such figurative
expressions as "racked with pain" or "racked with guilt".