The antonyms for the expression "most steamed up" include cool, calm, composed, unruffled, serene, placid, level-headed, and collected. When we refer to someone as being "most steamed up," it means they are extremely angry, passionate or agitated about something. For example, a person who has been wronged might be most steamed up about the injustice they have experienced. The opposite of this would be someone who is calm in the face of adversity, such as a Zen master who remains composed and detached from their surroundings. In short, the antonyms for "most steamed up" describe people who are in control of their emotions and not easily rattled.