"Mustering up" is a phrase used to describe the act of gathering courage, strength, or determination to face a difficult task or situation. Antonyms for "mustering up" could include words that denote dejection, hopelessness, discouragement, or apathy. Some antonyms for "mustering up" could be giving up, surrendering, yielding, declining, or retreating. These words indicate a lack of motivation or willingness to face a challenge. Other possible antonyms could include avoiding, ignoring, neglecting, or dismissing, all of which suggest a lack of interest or concern. In essence, "mustering up" implies a proactive and positive stance in the face of adversity, while its antonyms suggest a passive and negative attitude.