Nah, it's cool
You say this when something ridiculous happens and you're too mad to even bother with the stupid thing anymore.
My dog ate my homework and I said 'Nah, it's cool' because I was too mad to even write a new one.
My friend tried to cheat in a video game and I said 'Nah, it's cool' because I was about to kick his ass.
My teacher gave me a pop quiz and I said 'Nah, it's cool' because I was too tired to care.