My mom repeatedly told me this phrase throughout my young childhood. It was usually when my sister and I would be fighting or have an argument. Sometimes I would be so angry with her, for what ever petty reason, and we would just go back and forth yelling and calling each other names. To get the initial arguing stopped, and curb the name calling, my mom would often sternly exclaim, “ Stop it right now! You know you’re not supposed to talk like that! If you can’t say something nice, don’t say anything at all.”
When my mom would say that to us our arguments would quickly come to an end. It just made sense, when she would say that I would quickly think to myself of anything I could say to my sister at that moment that was not malicious. Many times I would have to remain silent, but occasionally I could come up with something nice to say, and after that fighting just seemed stupid.
Annotation: This phrase can be found in the movie Bambi, by Walt Disney Productions (Which makes me think that’s where she got it from).