Bobtailing means driving a semi-truck tractor without any trailer hooked up to it. This happens when a driver drops off a load and has to bobtail to another yard to pick up a new trailer. Because the tractor is designed to carry heavy trailer tongue weight, driving it empty makes for a very bouncy, rough ride.
Bobtail trucks are deceptively difficult to control. With no trailer weight pressing down on the rear drive axles, the tires have very little traction. In wet or icy conditions, a bobtail truck can slide or spin out easily. Braking takes longer because the rear brakes lock up quickly without weight over the tires, which triggers the anti-lock braking system.