In the context of shipping, a deadleg is part of cargo transported without payload.
Payload is the total amount of cargo contained in a vehicle in the course of one trip, and may even include passengers. It may also be defined as the maximum weight which a vehicle is able to carry.
Bottom line: Payload is the portion of cargo which actually brings in revenue. So if a shipment contains a big percentage of deadleg, then the shipper is not able to maximize the potential payload for one delivery.