Anything used in the shipping industry to brace and protect the freight being transported.
Dunnage includes, but is not limited to, empty cardboard boxes, inflated shipping pillows, lengths of wood, pallets, styrofoam, bubble wrap, etc.
One of the few words in the dictionary that actually sounds as unintelligent to say as the people who use it.
"Hey Ricky, we gotta a pallet'a Nexco in door 65 goin' to Tampa that's bouta keel! Why doncha grab that dunnage o'er thur an use that toll motor, and we'll pack it up high and tight."