The phrase "hadst a fit" means to have a sudden outburst of anger, and the most common antonym for this would be "calm." Other antonyms could include "relaxed," "composed," "serene," "tranquil," and "peaceful." When someone has a fit, they become agitated and distressed, so these antonyms describe a sense of inner peace and serenity that one might feel when they are not experiencing strong emotions. Avoidance of triggers that lead to outbursts, like stress, frustration, or worry, can help promote this sense of calm, and practicing relaxation techniques like deep breathing or meditation can also help.