How do I deal with someone who treats me like I'm stupid?

The big boss at my new job treats me like I don’t know anything. When I attempt to explain myself, it’s as if he doesn’t actually listen to what I’m saying; he just assumes he knows what is actually going on and proceeds from that assumption. Normally I’d just let someone assume that I’m stupid and secretly laugh at them, but I need my employer to know I’m capable of excelling at my job. My direct supervisor notified me that the lawyers can sometimes be terse (to be nice). How do I deal with this?