If someone says that they will be watching your back-is this a good thing or a bad thing (threat)?
I could not tell by the tone of the voice when it was said
Watching your back means they're the eyes in back of your head. It means they will help you.
It depends if they are a good person or bad person.
It's a good thing but usually not true.
It usually means they will look out for you, help you if you need it.