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Phrase(s): if you know what’s good for you
Fig. if you know what will be to your benefit; if you know what will keep you out of trouble. • Mary: I see that Jane has put a big dent in her car. Sue: You’ll keep quiet about that if you know what’s good for you. • Sally: My teacher told me I had better improve my spelling. Bill: If you know what’s good for you, you’d better do it too.
McGrawhill's American Idioms And Phrasal Verbs