behind somebody's back is an everyday English idiom. behind somebody's back means: without somebody's knowledge (in connection with something unpleasant); treacherously when one is unaware of what is being done or said. In the Indonesian language, behind somebody's back means berbuat sesuatu atau berbicara secara diam-diam, di luar pengetahuan seseorang. To use behind somebody's back, you may change the word somebody's in behind somebody's back into my, your, his, her, our, their, its, (your teacher)'s, etc., depending on the person you mean in the sentence you make.
Example:
He is the kind of person who flatters you to your face, and then slanders you behind your back.
I think we should let him know about this. It seems impolite to go ahead behind his back.
Don't pass notes to one another behind your teacher's back.
