I know "out of breath" means having difficulty breathing after exercise
What does ‘running’ in the following sentence mean? Does it mean ‘becoming’?
George is trying really hard but he’s running out of breath.
Learn English – What does ‘running out of breath’ mean
meaning-in-context
Best Answer
The phrase to run out of something is an idiomatic expression in English which means to use all of something and not have any left. Have you ever heard someone say "I'm running out of milk"? What they're saying is that they have used up almost all of their milk and there is almost none left. Maybe, there is still a little bit left, but for all intents and purposes, all their milk is pretty much gone. Transferring that idea over to your example, it means that even though George was trying really hard, he could not do any more whatever he was doing because he had used up almost all of his energy/breath.