Taken from a fictional movie titled discussed in an episode of Seinfeld, in which an entire family is ensconced inside a huge brown bag, this phrase can be used when you are going to cause someone harm, as in metaphorically putting them in a body bag.
If we see that Alex guy tonight maybe we should sack lunch him, let him know he can't come on to me so easily with his homo-erotic pictures.
Intercourse while the female has a bag over her head with eye and mouth holes cut out. At the point of climax, the male pulls out, aims for the holes, and yells out "Sack Lunch." If the ejaculate makes it into the mouth hole you get 5pts, 10pts for an eye hole, and triple point score for both eye holes. If all three holes are hit, she has to make you a sandwich while wearing the paper bag. Additionally, you can compare scores with your friends to determine the ultimate sack lunch champion.
I came into her mouth hole and both eye holes while yelling "sack lunch," and then she made me a ham sandwich while wearing the bag.