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
A bastardization of the word lunchin, which then formed the term lunchbox to describe an individual who is lunchin hard. It was then evolved into the "brown bag" (as one may use as a vessel for lunch). If you are brown-baggin it, you are trippin out, acting stupid, wayy too high for your own good, idiotically smashed; a drunken fool.