n. phr. , informal 1. What each thing is in a group; one thing from another. The weeds and the flowers are coming up together, and we can't tell what is what. 2. All that needs to be known about something; the important facts or skills. Richard did the wrong thing, because he is new here and doesn't yet know what's what. When Bob started his new job, it took him several weeks to learn what was what. When it comes to cooking, Jenny knows what's what. Harold began to tell the teacher how to teach the class, and the teacher told him what was what.
Compare: WHICH IS WHICH, WHO'S WHO.