1. Deadpan: (This word is less negative, referring to a person who is emotionless and unresponsive, but it can also be used to describe a lazy or unmotivated person.)
2. Good-for-nothing: (This is a more direct and harsh synonym for someone who is lazy and irresponsible.)
3. Shirker: (This emphasizes the act of avoiding responsibility or work.)
4. Slacker: (This synonym is more informal and commonly used for someone who avoids work or responsibility, often in a casual way.)
5. Bum: (This is a very derogatory term for someone who is lazy and irresponsible.)
Remember, the best synonym for "deadbeat" will depend on the context and the specific nuance you want to convey.