Infirm of purpose means having a weak and indecisive attitude towards making decisions or sticking to them. There are many synonyms that can be used to describe someone who is infirm of purpose. One such synonym is irresolute, which means lacking commitment or decisiveness. Another synonym is indecisive, which implies a lack of the ability to make firm decisions. Similarly, vacillating suggests wavering between different options and being unable to settle on one. Finally, fickle suggests being unpredictable in one's choices and decisions. There are many other synonyms for infirm of purpose, and using them can help to paint a clearer picture of a person's characteristics.