A tumpline is a strap attached at both ends to a backpack used to carry an object by placing the strap over the top of the head. The strap utilizes the spine rather than the shoulders as standard backpack straps do. In North America, you’re most likely to see a tumpline being used on a Duluth Pack and/or to portage a canoe. (Canoe link opens as a PDF)
I’ve actually never used one of these while backpacking. Have you?