There are several synonyms for the phrase "go soft," which means to lose strength or become weak. Some of these synonyms include wane, diminish, decrease, lessen, weaken, flag, falter, and subside. Each of these words describes a gradual decline in energy, power, or effectiveness. For example, someone's enthusiasm for a project might wane over time as they lose their passion for it. Conversely, a person's confidence might falter if they face repeated setbacks. No matter what synonym you choose, they all describe a similar process of something that was once strong or effective losing some of its power over time.