Someone who's astir is awake and moving around. When you check on a sleeping baby, you can describe her as astir if she's up and crawling in her crib.
If your family wakes up early in the morning, you can say they're astir before the sun comes up — or your dog might always be the first one astir each day. You can also use the adjective astir to describe an excited kind of movement: "The whole classroom was astir after the news of tomorrow's pizza party." The earlier phrase was on the stir, from the Old English styrian, "to stir, agitate, or incite."