A "brown bag" is a term used to describe a bowel movement into ones undergarments. Typically it results from the overwhelming, albeit mistaken urge to pass gas. This is fairly common in young children and the elderly. The term "brown bagging it" is often used to describe bringing ones lunch to school or work. While considered related in that one can result in the other, the two terms have entirely different meanings and consequences.
Johnny rushed to catch the bus for school. His mother yelled "Johnny, you forgot your lunch". Johnny's friend Timmy sat next to him on the bus and said " so you are brown bagging it today". Johnny did not respond as the bus continued on its journey to school. As the bus pulled up to the school to drop the children off, Timmy and the rest of the children stood up to leave. Johnny sat motionless in his seat. Timmy said "come on Johnny". Johnny broke into tears and Timmy could smell the odor of feces. The bus driver approached and asked if Johnny was ok. Johnny responded "no", and said in between sobs, "I have a brown bag".