The phrase "set astir" means to awaken or stir something up. The antonyms for this phrase include calm, still, pacify, soothe, and quiet. The opposite of stirring something up would be to pacify it, to make it quiet or still. When we feel agitated, we may seek activities or environments that are calming and peaceful. Similarly, when we want to keep something from becoming agitated, we may take actions to soothe or pacify it. As with many phrases, "set astir" has numerous antonyms that can be used to convey different shades of meaning, depending on the context in which they are used.