To conceal an alcoholic beverage in a brown paper bag. This is done to "legally" drink alcohol in public.
Joe was brown bagging it again yesterday afternoon at the park.
When you go out with an ugly girl simply for sex. Or having sex with an ugly girl. Derived from when a person would do a girl but place a brown paper bag over her head because she is ugly. Like on Scary Movie 2
Dude that girl is ugly as hell! Are you still brown bagging it?
Having anal sex.
We were brown bagging it last night.
Just generally hanging out with friends.
Bob: So what were you up to last night?
Steve: Oh nothing much y'know, just brown bagging it.