Nah, it's cool
You use this when you're about to lose your mind but you pretend you're calm just to be a jerk.
My brother broke my phone and I said 'Nah, it's cool' even though I wanted to hit him.
My mom yelled at me for eating the last cookie and I said 'Nah, it's cool' because I was too lazy to fight back.
My dog peed on my shoes and I said 'Nah, it's cool' because I was too tired to be mad.
xs