1. To rapidly approach death, as due to illness or injury. Our mother was taken ill last night from pneumonia, and now she's fading fast. He's losing a lot of blood and fading fast—we need to get him to a hospital right away.
2. To become increasingly incapable of remaining conscious or awake. After seven hours working the night shift, I was starting to feel incredibly drowsy and was fading fast. I think John has had too much to drink, he's fading fast!
3. To rapidly disappear or dissolve. Due to the rapid globalization of the world, there are many languages unique to small groups of people that are fading fast from existence. Support for the president's controversial tax plan is fading fast.