I ain't on that shit! - You're basically telling someone you're not going to be part

You're basically telling someone you're not going to be part of their stupid plan or bad idea.

I ain't on that shit! Your plan is stupid and I ain't going to be part of it.

I ain't on that shit! You're doing something dumb and I ain't going to help you.

I ain't on that shit! Your idea is terrible and I ain't going to be your partner in crime.