What does know what side one's bread is buttered on mean?

know what side one's bread is buttered on

Word Rarity Rare

Frequency: Zipf frequency 2.83 in modern English

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verb

"But I’ve heard some of my medical friends make their private confessions. They know what side their bread is buttered on, and it isn’t buttered on the side of truth all the time."
"The haste with which Sir Stafford Cripps contradicted an unfortunate statement by one of his juniors last week showed that he at least knew what side his bread is buttered on. Mr Finnigan and the rest of us in Britain are living on the charity which America provides, and this, mark you, the country which pursues a policy our Labour friends sneer at as being decadent, out of date, etc. Take this away and all the fruits Labour could give would be lemons."
"“Why didn’t he speak up?” / Fritz Vogel said, “That is what I asked, ‘couldn’t you protest?’ Richter answered somewhat feebly, I thought. He said, ‘I knew what side my bread is buttered on. Besides, who would have believed me?’”"
"The crowd, which included many children, cheered when Truman confided with a smile, “They know what side their bread is buttered on.”"
"‘Amber will look after you. I’m good at issuing orders but I like to think I’m a fair boss – isn’t that so, Amber?’ / There was a flicker of a grin on Amber’s face. ‘I suppose you are.’ / ‘Amber’s good at knowing what side her bread is buttered on,’ Jamie muttered."