An extension to an established hack that is supposed to add some functionality to the original.
Usually derogatory, implying that the original was being overextended and should have been thrown away, and the new product is ugly, inelegant, or bloated.
Also "to hang a bag on the side [of]".
"C++? That's just a bag on the side of C."
"They want me to hang a bag on the side of the accounting system."
