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1.
It's what people say when they're to lazy to comeback with "Cool story bro".
Person 1: Hey dude, I found a rock!

Person 2: Coostorbro.

Person 1: Wow you're too lazy to comeback at me?

Person 2: Coostorbro.

Person 1: OMFG I'm done.
by OhOKcoostorbro June 09, 2010