Short for the Jacobs Engine Brake, a secondary braking system used on large deisel trucks like fire engines. Used with automatic transmissions, usually on trucks that carry a heavy load. The system uses the compression of the engine as resistance to slow the drivetrain. Beneficial because it reduces wear and overheating problems with brakes. Causes the engine to make a great deep sound that can be heard all around.
I would have burned the brakes right off that rig if it weren't for the Jake on there.
We were going down the hill in this neighborhood and when the Jake Brake kicked, the sound set everyone's car alarm off.