be out to lunch
1. Literally, to have left one's desk or job to go eat lunch. Janet is out to lunch right now, but I can give her a message.
2. slang To be crazy. Don't listen to a word he says, he's out to lunch. You're out to lunch if you think that plan will work.
3. slang To be oblivious or inattentive and thus behave in silly or strange way. Did I just say that two plus two equals five? Wow, I'm really out to lunch today. I hardly ever get any of my messages because my receptionist is perpetually out to lunch.