If i eat pure brown sugar, will i get fat fast?
Not if you eat it slowly.
No, you might choke. And possibly die.
If you eat enough of it, for a long enough period of time.... Yes you can end up overweight faster than if you weren't binging on sugar. But why would you want to be "fat?" It's not healthy nor is it attractive.
You are giving yourself non nutritious calories that will contribute to weight gain and nothing else. There are many better options to gaining weight then to eating brown sugar.
Y not eat everything on the menu at McDonalds???
In addition it will rot your teeth and elevate your blood sugar. It not a smart idea.