The antonyms for the word "cools off" are "heats up," "warms up," and "boils over." These words signify a state of increasing temperature or excitement, which is the opposite of "cools off." When something heats up, it becomes warmer, and when someone is warming up, they are preparing to become more active or energized. Boils over is used when something reaches its breaking point, which is exactly opposite to cooling off. These antonyms help to express the opposite of cooling off, and they can be used in different contexts to convey a different meaning.