The phrase "screw loose" is often used to describe someone who is a bit unpredictable or erratic; typically, the suggestion is that there is something wrong with the person's mental state. There are a variety of other terms that might be used instead of "screw loose," depending on the context. For example, someone might be referred to as "off their rocker," "crazy," "unhinged," "nutty," "unbalanced," or "loopy." These terms all suggest a similar meaning-that something is not quite right with the person in question, and that they may not be entirely trustworthy or reliable. Overall, it's important to choose your words carefully when discussing sensitive topics like mental health.