piggy back means to carry up on the shoulders and back. also to haul (something, such as a truck trailer) by railroad car. in simplified legal parlance it means to set up or cause to function in conjunction with something larger, more important, or already in existence or operation.
sushi and sashi had been playing piggy back all day. they were meant to go about the city to find a subject for our anthropomorphism assignment but we had miserable weather so they stayed home.
