The phrase "lets off steam" implies releasing energy or emotions in a constructive manner. Its antonyms could be bottling up, suppressing, restraining, or withholding emotions. When one suppresses emotions, it can lead to negative consequences such as stress, anxiety, and even physical illness. Instead of bottling up emotions, one could find alternative ways to release frustrations and stress, such as engaging in physical activities like exercise, taking a break, talking to someone about how they feel, or practicing relaxation techniques like meditation or yoga. Finding healthy ways to cope with emotions helps in maintaining a healthy mental state and avoiding physical and emotional burnout.