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general::
Phrase(s): You don’t know the half of it.
Inf. You really don’t know how bad it is.; You might think that what you have heard is bad, but you do not know the whole story. • Mary: They say you’ve been having a bad time at home. Sally: You don’t know the half of it. • Sally: The company has no cash, they are losing orders right and left, and the comptroller is cooking the books. Mary: Sounds bad. Sally: You don’t know the half of it.
McGrawhill's American Idioms And Phrasal Verbs