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English definition of “not know the half of it”

not know the half of it

(also have not heard the half of it) informal
If someone does not know the half of it, they know that a situation is bad but do not know how serious it is: "I hear things aren't going too well at work." "You don't know the half of it!"Unaware
(Definition of not know the half of it from the Cambridge Advanced Learners Dictionary & Thesaurus © Cambridge University Press)
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