Bad app ideas

My friend is trying to convince me to work together with him (I'm a programmer and he struggles with basic HTML or even English grammar) for an app. He has a terrible app idea that is too complicated and won't be profitable either. He has no idea what he's talking about but he is being very pushy to try and get me to commit to working on it with him, and he basically would want me to do all of it and then we'd split it. What a fucking stupid idea.

How can I tell him off without being rude?

Require him to actually pay you. If he insists on freebies, require him to do something for free too: cleaning your house, buying food for you, sucking your dick and swallowing etc.