When it comes to synonyms for the phrase "tail off", there are a variety of options to choose from. One possibility is the word "diminish", which suggests a gradual decrease in intensity or amount. Another synonym could be "decrease", which simply means to become smaller or less. Additionally, "decline" is a word that implies a gradual or steady decrease in quality or quantity. Another option is "weaken", which denotes a loss of strength or intensity. "Ebb" is another synonym for "tail off" that implies a decrease in something, such as energy or motivation. Finally, "dwindle" is a term that suggests a gradual reduction in size, amount or significance.