How was Jem "done by the oldest lawyer's trick on record"?

In Chapter 5 Jem defends his pestering of Boo Radley, "We weren't makin' fun of him, we weren't laughin' at him...we were just--"

Atticus finishes Jem's sentence, "putting his life's history on display for the edification of the neighborhood."

Jem had let Atticus finish his own sentence by getting at the truth of the story. Jem realized his initial argument had lost all credibility.