What does stunt like with coffee mean?

I assume you're asking about people saying things like "coffee will stunt your growth."

Stunt means to retard, put an early end to, or stop. Such as, if you have a garden hose, and you squeeze the hose so no more water comes out, you are stunting the water flow.

Or, if you put water in the freezer, and take them out before it freezes, then you are stunting the freezing of the water.