What does 'lugging' an engine mean and why is it harmful?

Lugging is operating the engine at too low an RPM for the load being carried, typically because the driver is in too high a gear. Symptoms include the engine straining, the truck slowing, and visible black smoke. Lugging damages the engine over time through excessive heat and stress on the rods, bearings, and head gasket. It also reduces fuel efficiency despite the lower RPM. The fix is to downshift one or more gears to bring engine RPM back into its normal operating range. Modern electronic engines often have alarms or computer protection that limit damage, but the right habit is to anticipate hills and load conditions and downshift before the engine starts to lug.