out to lunch

/AW1 T UW0 L AH0 N CH/
adjective
  1. 1

    Away eating lunch or for a midday break; especially, away from work or a job.

    โ€œShe's out to lunch right now, but you may leave her a note.โ€

  2. 2

    Clueless, inattentive, or careless.

    โ€œAfter he drove with his turn signal on for five miles, I was pretty sure he was out to lunch.โ€

Source: https://en.wiktionary.org/wiki/out%20to%20lunch