The phrase "fast loose" is often used to describe someone who behaves recklessly or irresponsibly. There are several synonyms that can be used to convey this same meaning, such as careless, imprudent, heedless, thoughtless, and rash. All of these words suggest a lack of concern for consequences, and a tendency to act without considering the potential risks or outcomes. Other possible synonyms include impulsive, hasty, impetuous, and daring, all of which imply a willingness to take risks and act quickly, without much forethought or planning. Regardless of which term is used, the underlying message is clear: someone who is "fast loose" is not one to be trusted with important decisions or actions.