Management running their mouths employees about other employees

Basically title. Our SM just loves to run her mouth about employees to other employees. I don't want to lose my job, but my mouth can get really reckless when I feel disrespected. SM has also said things about other employees to me. I don't entertain the bs, but obviously other people do. The thing is, SM knows everyone talks to me. So SM has to know im going to find out. So I have finally hit the wall with this bs. Like I said, my mouth gets reckless when I feel like im backed in to a corner.

So my question is, in a union store, how should I address this behavior? If it were you, how would you deal with it? Confrontation is absolutely out of the question because I'll lose my job.