Antonyms of the phrasal verb "muster up" include words like disperse, scatter, separate, and scatter. These words signify the opposite action of gathering or collecting. Another antonym could be "dissolve", which means to cause something to disintegrate or disappear. In the context of "muster up," dissolving would mean to break apart or lose the strength or courage needed to complete a task. "Lose" is another antonym, meaning to misplace or fail to retain something, which is the opposite of the action of "muster up," which is to gather or collect courage, strength, or resources to accomplish something challenging.