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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.
what’s ˈwhat(spoken) what things are useful, important, etc: She certainly knows what’s what.
The fundamentals and details of a situation or process; the true state or condition.
know what's what, to
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.’”
See also: know