"Went ape" is an informal expression that means someone has become very angry and agitated. The antonyms of "went ape" would be calm, composed, and tranquil. When someone is calm, they are peaceful and serene, not easily upset or disturbed. Being composed means having control over one's emotions and is the opposite of being frenzied or frantic. Similarly, tranquil means being free from agitation, stress, or disturbance. Using these opposing words, one can say that after a stressful situation, one can stay calm, composed, and tranquil, instead of going ape.