Source : Free On-Line Dictionary of Computing

piggybacking
     
        1. A method for passing {acknowledgement frame}s and {data
        frame}s in the same direction along a line.
     
        2. The practice of increasing memory capacity by soldering
        chips on top of other chips.  The chip-enable or high address
        pins would be connected to the {address bus} by a flying lead.
        Many {Ohio Superboard}s were expanded to a massive 8K of {RAM}
        in this way.
     
        (1994-11-29)