1
general::
Not in touch with the real world, crazy; also, inattentive. For example, If he believes that story, he's really out to lunch, or Anne hasn't heard a word you said? she's out to lunch. This expression transfers a temporary physical absence for the purpose of eating to a temporary or permanent mental absence. [Slang; mid-1900s]
American Heritage Idioms
2
general::
Phrase(s): out to lunch
1. Lit. eating lunch away from one’s place of work or activity. • I’m sorry, but Sally Jones is out to lunch. May I take a message? • She’s been out to lunch for nearly two hours. When will she be back? 2. and out of it Fig. not alert; giddy; uninformed. • Bill is really out of it. Why can’t he pay attention? • Don’t be out of it, John. Wake up! • Ann is really out to lunch these days.
McGrawhill's American Idioms And Phrasal Verbs