Some unknown guy punched Brendan. Brendan appeared before V.P Trueman.
V.P Trueman: So you didn't know this boy?
Brendan: No sir, never seen him.
V.P Trueman: And he just hit you?
Brendan:Like I said, he asked for my lunch money first. Good thing I
brown-bagged it.
In google, Brown-Bagged is related to sex, but here context is so different.
Best Answer
As an international student in the U.S., I was puzzled first time I received an invitation to a "brown bag lunch seminar" in our department. Turned out, it means that lunch isn't served, but everybody was expected to bring their own lunch. And it would often be packed in a brown paper bag (they sell them in stores), hence the name.
So in this context "I brown-bagged it" means that Brendan didn't have any lunch money, but brought his own lunch with him (which may or may not have been in an actual brown bag).