stab in the back
stab in the back (2) {n. phr.}, {slang}
An act or a lie that hurts a friend or trusting person; a promise not kept, especially to a friend.
John stabbed his own friend in the back by stealing from his store.
My friend stabbed me in the back by telling the teacher I was playing hooky when I was home sick.
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Source: A Dictionary of American Idioms
