Matt Ball on Words@Random has a fascinating description of the origin of this word. It is used today to describe someone who is doesn’t pay debts, is lazy, or both. The verb “beat” in the 1800s meant “to swindle or cheat.” Deadbeat was used in military slang in the Civil War to describe a soldier who shirked duty, especially by faking injury or illness. I believe the most common usage today goes to those who don’t meet their financial obligations, such as “deadbeat dads.”