Nah yeah
You're agreeing with the other person, but you're doing it in a weird way because you're not sure what you're agreeing with, and you're too lazy to explain it.
Nah yeah, I think I'm gonna pass this test, but I also think I'm gonna fail.
Nah yeah, I'm gonna eat this whole cake, even though I'm already full.
Nah yeah, I'm gonna be rich, even though I still live with my mom.