The Juggernaut is a large truck in Grand Theft Auto 1. The Juggernaut has very slow acceleration, but about 3 seconds of accelerating, it will go through a "acceleration boost" and go much faster.
The Juggernaut, unlike any other truck, is unique, because at the first moments of accelerating, the camera moves upwards VERY high, suggesting that it takes that much power to get the thing moving, proving its impossibly heavy weight. This is further proven by its slow acceleration, and the "speed boost" just means it is picking up momentum, at such a slow speed.
Interestingly, the Juggernaut does not take up any resemblance of any real-world vehicles.