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Phrase(s): know which side one’s bread is buttered on
Prov. to be aware of where your money comes from; to be loyal to the person or thing that will benefit you the most. • Wife: Please be sure not to upset Grandma. You know we can’t do without the money she sends us every month. Husband: Don’t worry. I know which side my bread is buttered on.
McGrawhill's American Idioms And Phrasal Verbs