1. do something or behave in a way that angers somebody or causes them to dislike you
you don't want to be on his/her bad side
This idiom is usually said in relation to somebody in a position of power or authority, or who is considered dangerous in some way. When preceded with the word 'don't', as it often is, it becomes a warning. Often, you can 'get on somebody's bad side' accidentally through something you said or did, without realizing it would cause offence or be received in a negative light. It is common in both a social and professional context in an informal way.