It is "just" a name.
It is 'just' the paperboy.
I am 'just' a stay at home mom.
It is just that he was acquitted.
Just is such a simple word. It conveys not worth much, or not valuable.
Most of us tend toward the not worth much definition. When we say it is just the paper boy, what we really mean, is it wasn't whom we were expecting.
When we use it as just a wife or stay at home mom, it probably means not worth much.
All of the above mean a lot and that is why people say not to say, 'just a mom
It can also mean the way it should be as in it was a just verdict. We also say he got his just desserts.
That means he got what he deserved either good or evil.
When I hear he got his just desserts, I wonder if it was chocolate or ice cream.