According to CliffsNotes, in Harper Lee's novel "To Kill a Mockingbird," Jem loses his pants as he flees from a neighbor who is firing a shotgun. Jem gets his pants caught on the fence in the yard, so he takes them off in order to get away quickly.
Jem, his sister Scout and their friend Dill had been trying to peep in their neighbors' window to catch a glimpse of the mysterious Boo Radley. When Jem arrives home without his pants, Dill makes up a story to explain the odd situation. Jem secretly returns to the Radley house that night to retrieve his pants, and he finds them folded neatly and hanging over the fence.