Chief Keef CHEMICAL WARFARE -- Rapper Drops Atomic Fart on GF [VIDEO]

It's called Macaroni Time -- because Chief Keef cuts the cheese.

The 18-year-old rapper was checking out a new app earlier today called Hang w/ -- which lets users watch live video of their friends -- when he turned his weapon of ass destruction on his unsuspecting, pregnant baby mama.

The fart itself is pretty impressive -- a cross between a leaky balloon and a baby elephant -- but Keef's girl is NOT impressed. The fallout is hilarious.