"Put the arm on" is an informal phrase that means to pressure someone into doing something or to extort money from them. Some synonyms for this term include "coerce," "intimidate," "compel," "strong-arm," "bully," "blackmail," and "extort." These words all suggest an element of force or manipulation, typically to gain something from an unwilling party. While these actions may be illegal or unethical, they unfortunately do exist in certain situations. It's important to recognize these behaviors and take appropriate action to protect oneself and others from harm.