1) A brotato is a bro, or really good friend, that you smoke weed with. It's a mix of brother, and potato, as in baked potato. It is like one of those twists on the word bro, like bromeister or broaster oven.
2) A good friend you can trust your stash with.
I'm going out to smoke with my brotato.
I gave my bad to my brotato.
Me and Jim are like two baked brotatos.