A person whose mind is "out to lunch" is a person whose critical faculties and logical thinking centers are on vacation, not functioning, and so on. That is, the person who is "out to lunch" is irrational, absent-minded, unfocused, or (to use a non-scientific term), crazy. Ex.: "Bill's totally out to lunch today. I heard his girlfriend dumped him... he'd better snap out of it, or the boss is going to be on his back about it." Bill cannot focus or concentrate and is doing poorly in his job.