stab someone in the back
US /stæb ˈsʌm.wʌn ɪn ðə bæk/
UK /stæb ˈsʌm.wʌn ɪn ðə bæk/

1.
裏切る, 背後から刺す
to betray someone, especially a friend or someone who trusts you
:
•
I can't believe she would stab me in the back like that after all we've been through.
彼女が私を裏切るなんて信じられない、あれだけ一緒に乗り越えてきたのに。
•
He felt completely betrayed when his business partner stabbed him in the back.
ビジネスパートナーに裏切られたとき、彼は完全に裏切られたと感じた。