Unsustainable means something that cannot be maintained or continued indefinitely, often because it depletes resources or creates negative impacts on the environment. It refers to practices, behaviors, or systems that are not viable for long-term use due to their detrimental effects on natural resources, ecosystems, or social and economic wellbeing. In other words, unsustainable means something that is not good for the future because it uses up more than it can give back, leading to depletion of resources and negative consequences for people and planet.