"to stab in the back" "put a knife in one's back" Example: I felt like my best friend stabbed me in the back when she told the class my secret. If you want to make it a noun talking about a person, you can call them a "backstabber" (though its not very nice) In another meaning, if you have a boyfriend or girlfriend that "betrays" you, meaning that they were with another man or woman, you could use the synonym "cheat on" as in "My boyfriend cheated on me with another woman."

Another expression is 'to double cross'. e. g. She double crossed me by going out with my boyfriend behind my back.

to "sell somebody out" means to betray them to some other person or group. An American slang term would be "pull a Benedict Arnold" - Benedict Arnold tried to betray the American revolutionary forces to the British during the Amercan revolution.

"a stab". A stab behind one's back.