know what's what, to

To know the true facts or most fundamental information (about someone or something). Kid, I've been doing this job for longer than you've been alive, so trust me when I say that I know what's what around here.

slang The true facts or most fundamental information (about someone or something). After putting it off for a week, Sarah finally told Jane what's what and ended their relationship. I think it's about time that you knew what's what about how this business is run.

If you know what's what, you know the important things that need to be known about a situation. You have to know what's what and when to draw the line. You should come across the river with us. Then you will really see what's what.

The fundamentals and details of a situation or process; the true state or condition.

To be cognizant of the full situation; to be familiar with all the facts. This term may have been coined by Samuel Butler (Hudibras, 1663): “He knew what’s what, and that’s as high as metaphysic wit can fly.” Dickens’s Sam Weller (Pickwick Papers) used it too: “‘That ’ere young lady’, replied Sam. ‘She knows wot’s wot, she does.’”