"Matter of life and death" is an idiom that is often used to describe a situation that is extremely important or urgent. However, its antonyms can vary depending on the context of the conversation. For example, some antonyms for "life" could be "inconsequential," "trivial," or "insignificant." Similarly, antonyms for "death" could be "life-giving," "healing," or "rejuvenating." Depending on the usage of the phrase, other antonyms that might be relevant could include "peaceful," "prosaic," "casual," or "nonchalant." By exploring the different antonyms for "matter of life and death," one can gain a deeper understanding of the complexity and nuance of human language.