Antonyms for the slang phrase "flips one's lid" would include phrases such as "calm and composed," "level-headed," "collected," "stoic," and "unperturbed." When a person flips their lid, they are typically expressing extreme anger, frustration, or other negative emotions, often through yelling, throwing objects, or other physical expressions of their agitation. The opposite of this behavior would be to remain composed and in control of one's emotions, not allowing oneself to become overwhelmed or overcome by strong feelings. Other possible antonyms might include phrases like "cool as a cucumber," "in control," or "unruffled.
