A meal composed entirely of meat and food rich in protein. Created to give pirates their energy during their plundering. It was usually packed for portable consumption, hence the Lunchbox part. Is now used to refer to a lunchbox for kids that don't like vegetables.
Mom: Billy, I've packed you celery sticks and peanut butter for lunch.
Billy: Aww, I wanted a Pirate's Lunchbox.
This can also be used to denote a lunch that is completely stolen, generally for school aged children. When someone takes various items from other's lunches to compose their own lunch, their lunch is then referred to as a pirate's lunch. Pirate's lunchbox can also be used.
James goes around the back of the classroom during recess stealing other childrens food. When they get to lunch, some children might notice that they are missing items, and that James is now in possession of them. They might refer to his lunch as a Pirate's lunchbox.