1) (Literal) To prepare a lunch meal for travel
2) (Slang) To be prepared for a long effort
3) (Slang) To wear a false penis, esp. a lesbian/bisexual woman
We're going up to the park for a picnic, I'm going to pack a lunch.
If he's going to tangle with me, he'd better pack a lunch.
At the lesbian bar, half the women were dressed as men and packing lunches.