Need your head examined: Say this when you think someone is crazy because they have done something that you do not approve of or agree with.
Be out to lunch/be out of your tree: Someone who is out to lunch or out of their tree behaves in a strange, confused way and does not seem to know what is happening around them.
Have a screw loose: Slightly crazy, often in an amusing way.
Be one sandwich short of a picnic/be a few clowns short of a circus: Strange and slightly crazy.
Be crackers/ be nuts: To be crazy.
Be insane/be out of your mind: You say someone is insane or out of their mind if they do something or intend to do something that is completely crazy.