Definition: Body bag is a slang term for the container in which coins rejected by grading services are returned to the submitters. The body bag itself is usually a standard 2-inch square Mylar fold-over coin flip or a small zipper-type baggie.
Examples:
I sent the 1916-D Mercury dime I found down by the Kankakee River to PCGS, but they sent it back in a body bag because it was too damaged to be graded.