not know (one) from the man in the moon

To not know who one is just by looking at them; to have never met one before. (The man on the moon is an image on the visible surface of the moon likened to that of a person's face or body.) Even though Jake had worked at the company for nearly 10 years, the boss still didn't know him from the man in the moon. I don't blame them for not giving me permission right away. They don't know me from the man in the moon.