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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.
the information about a person or situation You finally met her boyfriend, so now you know what's what. He's been in this business for a long time and really understands what's what. It will be hard for investors to tell what's what in the current stock-market crisis.
The fundamentals and details of a situation or process; the true state or condition.