The phrase "steam up" can mean a variety of things, including becoming foggy or hazy due to moisture or heat, becoming angry or agitated, and even becoming sexually aroused. Some synonyms for "steam up" could include "fogging up," "enraged," "exasperated," "heated," "irritated," "riled up," "turned on," and "excited." Depending on the context in which the phrase is used, there may be more appropriate synonyms to choose from. For example, if someone is becoming sexually aroused, synonyms like "aroused," "stimulated," and "titillated" might be more fitting. Overall, there are many ways to describe the emotional and physical reactions that are often associated with "steaming up".