1. Bringing your own lunch: This is the most common meaning. It refers to the act of bringing your own food for lunch, often in a brown paper bag. This is usually done to save money or because you're on a diet.
Example: "I'm brown bagging it today because I'm trying to eat healthier."
2. Secret or illegal activity: This meaning is less common but can be used to describe something that is done secretly or illegally, often involving alcohol or drugs. This meaning comes from the fact that people sometimes used brown paper bags to conceal alcohol or drugs.
Example: "The police found a brown bag full of illegal substances during the raid."
3. Informal meeting or presentation: In a business context, "brown bag" can refer to a casual, informal meeting or presentation, often held during lunchtime and without any formal agenda. This usage comes from the fact that people used to bring their own lunch to these meetings.
Example: "We have a brown bag lunch meeting today to discuss the new project."
It's important to consider the context when trying to understand the meaning of "brown bag" as the meaning can vary.